40e47125e6
Signed-off-by: Masanari Iida <standby24x7@gmail.com> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2201 lines
68 KiB
Plaintext
2201 lines
68 KiB
Plaintext
[Generated file: see http://ozlabs.org/~rusty/virtio-spec/]
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Virtio PCI Card Specification
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v0.9.1 DRAFT
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-
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Rusty Russell <rusty@rustcorp.com.au>IBM Corporation (Editor)
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2011 August 1.
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Purpose and Description
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This document describes the specifications of the “virtio” family
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of PCI[LaTeX Command: nomenclature] devices. These are devices
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are found in virtual environments[LaTeX Command: nomenclature],
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yet by design they are not all that different from physical PCI
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devices, and this document treats them as such. This allows the
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guest to use standard PCI drivers and discovery mechanisms.
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The purpose of virtio and this specification is that virtual
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environments and guests should have a straightforward, efficient,
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standard and extensible mechanism for virtual devices, rather
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than boutique per-environment or per-OS mechanisms.
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Straightforward: Virtio PCI devices use normal PCI mechanisms
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of interrupts and DMA which should be familiar to any device
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driver author. There is no exotic page-flipping or COW
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mechanism: it's just a PCI device.[footnote:
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This lack of page-sharing implies that the implementation of the
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device (e.g. the hypervisor or host) needs full access to the
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guest memory. Communication with untrusted parties (i.e.
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inter-guest communication) requires copying.
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]
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Efficient: Virtio PCI devices consist of rings of descriptors
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for input and output, which are neatly separated to avoid cache
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effects from both guest and device writing to the same cache
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lines.
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Standard: Virtio PCI makes no assumptions about the environment
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in which it operates, beyond supporting PCI. In fact the virtio
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devices specified in the appendices do not require PCI at all:
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they have been implemented on non-PCI buses.[footnote:
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The Linux implementation further separates the PCI virtio code
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from the specific virtio drivers: these drivers are shared with
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the non-PCI implementations (currently lguest and S/390).
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]
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Extensible: Virtio PCI devices contain feature bits which are
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acknowledged by the guest operating system during device setup.
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This allows forwards and backwards compatibility: the device
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offers all the features it knows about, and the driver
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acknowledges those it understands and wishes to use.
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Virtqueues
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The mechanism for bulk data transport on virtio PCI devices is
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pretentiously called a virtqueue. Each device can have zero or
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more virtqueues: for example, the network device has one for
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transmit and one for receive.
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Each virtqueue occupies two or more physically-contiguous pages
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(defined, for the purposes of this specification, as 4096 bytes),
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and consists of three parts:
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+-------------------+-----------------------------------+-----------+
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| Descriptor Table | Available Ring (padding) | Used Ring |
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+-------------------+-----------------------------------+-----------+
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When the driver wants to send buffers to the device, it puts them
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in one or more slots in the descriptor table, and writes the
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descriptor indices into the available ring. It then notifies the
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device. When the device has finished with the buffers, it writes
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the descriptors into the used ring, and sends an interrupt.
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Specification
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PCI Discovery
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Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000
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through 0x103F inclusive is a virtio device[footnote:
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The actual value within this range is ignored
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]. The device must also have a Revision ID of 0 to match this
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specification.
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The Subsystem Device ID indicates which virtio device is
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supported by the device. The Subsystem Vendor ID should reflect
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the PCI Vendor ID of the environment (it's currently only used
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for informational purposes by the guest).
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+----------------------+--------------------+---------------+
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| Subsystem Device ID | Virtio Device | Specification |
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+----------------------+--------------------+---------------+
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+----------------------+--------------------+---------------+
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| 1 | network card | Appendix C |
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+----------------------+--------------------+---------------+
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| 2 | block device | Appendix D |
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+----------------------+--------------------+---------------+
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| 3 | console | Appendix E |
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+----------------------+--------------------+---------------+
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| 4 | entropy source | Appendix F |
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+----------------------+--------------------+---------------+
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| 5 | memory ballooning | Appendix G |
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+----------------------+--------------------+---------------+
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| 6 | ioMemory | - |
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+----------------------+--------------------+---------------+
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| 9 | 9P transport | - |
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+----------------------+--------------------+---------------+
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Device Configuration
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To configure the device, we use the first I/O region of the PCI
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device. This contains a virtio header followed by a
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device-specific region.
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There may be different widths of accesses to the I/O region; the “
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natural” access method for each field in the virtio header must
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be used (i.e. 32-bit accesses for 32-bit fields, etc), but the
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device-specific region can be accessed using any width accesses,
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and should obtain the same results.
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Note that this is possible because while the virtio header is PCI
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(i.e. little) endian, the device-specific region is encoded in
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the native endian of the guest (where such distinction is
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applicable).
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Device Initialization Sequence
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We start with an overview of device initialization, then expand
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on the details of the device and how each step is preformed.
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Reset the device. This is not required on initial start up.
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The ACKNOWLEDGE status bit is set: we have noticed the device.
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The DRIVER status bit is set: we know how to drive the device.
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Device-specific setup, including reading the Device Feature
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Bits, discovery of virtqueues for the device, optional MSI-X
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setup, and reading and possibly writing the virtio
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configuration space.
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The subset of Device Feature Bits understood by the driver is
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written to the device.
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The DRIVER_OK status bit is set.
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The device can now be used (ie. buffers added to the
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virtqueues)[footnote:
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Historically, drivers have used the device before steps 5 and 6.
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This is only allowed if the driver does not use any features
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which would alter this early use of the device.
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]
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If any of these steps go irrecoverably wrong, the guest should
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set the FAILED status bit to indicate that it has given up on the
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device (it can reset the device later to restart if desired).
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We now cover the fields required for general setup in detail.
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Virtio Header
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The virtio header looks as follows:
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+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
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| Bits || 32 | 32 | 32 | 16 | 16 | 16 | 8 | 8 |
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+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
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| Read/Write || R | R+W | R+W | R | R+W | R+W | R+W | R |
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+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
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| Purpose || Device | Guest | Queue | Queue | Queue | Queue | Device | ISR |
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| || Features bits 0:31 | Features bits 0:31 | Address | Size | Select | Notify | Status | Status |
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+------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
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If MSI-X is enabled for the device, two additional fields
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immediately follow this header:
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+------------++----------------+--------+
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| Bits || 16 | 16 |
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+----------------+--------+
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+------------++----------------+--------+
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| Read/Write || R+W | R+W |
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+------------++----------------+--------+
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| Purpose || Configuration | Queue |
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| (MSI-X) || Vector | Vector |
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+------------++----------------+--------+
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Finally, if feature bits (VIRTIO_F_FEATURES_HI) this is
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immediately followed by two additional fields:
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+------------++----------------------+----------------------
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| Bits || 32 | 32
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+------------++----------------------+----------------------
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| Read/Write || R | R+W
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+------------++----------------------+----------------------
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| Purpose || Device | Guest
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| || Features bits 32:63 | Features bits 32:63
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+------------++----------------------+----------------------
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Immediately following these general headers, there may be
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device-specific headers:
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+------------++--------------------+
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| Bits || Device Specific |
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+--------------------+
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+------------++--------------------+
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| Read/Write || Device Specific |
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+------------++--------------------+
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| Purpose || Device Specific... |
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| || |
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+------------++--------------------+
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Device Status
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The Device Status field is updated by the guest to indicate its
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progress. This provides a simple low-level diagnostic: it's most
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useful to imagine them hooked up to traffic lights on the console
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indicating the status of each device.
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The device can be reset by writing a 0 to this field, otherwise
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at least one bit should be set:
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ACKNOWLEDGE (1) Indicates that the guest OS has found the
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device and recognized it as a valid virtio device.
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DRIVER (2) Indicates that the guest OS knows how to drive the
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device. Under Linux, drivers can be loadable modules so there
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may be a significant (or infinite) delay before setting this
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bit.
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DRIVER_OK (3) Indicates that the driver is set up and ready to
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drive the device.
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FAILED (8) Indicates that something went wrong in the guest,
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and it has given up on the device. This could be an internal
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error, or the driver didn't like the device for some reason, or
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even a fatal error during device operation. The device must be
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reset before attempting to re-initialize.
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Feature Bits
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The least significant 31 bits of the first configuration field
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indicates the features that the device supports (the high bit is
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reserved, and will be used to indicate the presence of future
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feature bits elsewhere). If more than 31 feature bits are
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supported, the device indicates so by setting feature bit 31 (see
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[cha:Reserved-Feature-Bits]). The bits are allocated as follows:
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0 to 23 Feature bits for the specific device type
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24 to 40 Feature bits reserved for extensions to the queue and
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feature negotiation mechanisms
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41 to 63 Feature bits reserved for future extensions
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For example, feature bit 0 for a network device (i.e. Subsystem
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Device ID 1) indicates that the device supports checksumming of
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packets.
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The feature bits are negotiated: the device lists all the
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features it understands in the Device Features field, and the
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guest writes the subset that it understands into the Guest
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Features field. The only way to renegotiate is to reset the
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device.
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In particular, new fields in the device configuration header are
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indicated by offering a feature bit, so the guest can check
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before accessing that part of the configuration space.
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This allows for forwards and backwards compatibility: if the
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device is enhanced with a new feature bit, older guests will not
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write that feature bit back to the Guest Features field and it
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can go into backwards compatibility mode. Similarly, if a guest
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is enhanced with a feature that the device doesn't support, it
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will not see that feature bit in the Device Features field and
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can go into backwards compatibility mode (or, for poor
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implementations, set the FAILED Device Status bit).
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Access to feature bits 32 to 63 is enabled by Guest by setting
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feature bit 31. If this bit is unset, Device must assume that all
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feature bits > 31 are unset.
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Configuration/Queue Vectors
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When MSI-X capability is present and enabled in the device
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(through standard PCI configuration space) 4 bytes at byte offset
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20 are used to map configuration change and queue interrupts to
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MSI-X vectors. In this case, the ISR Status field is unused, and
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device specific configuration starts at byte offset 24 in virtio
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header structure. When MSI-X capability is not enabled, device
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specific configuration starts at byte offset 20 in virtio header.
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Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
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Configuration/Queue Vector registers, maps interrupts triggered
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by the configuration change/selected queue events respectively to
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the corresponding MSI-X vector. To disable interrupts for a
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specific event type, unmap it by writing a special NO_VECTOR
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value:
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/* Vector value used to disable MSI for queue */
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#define VIRTIO_MSI_NO_VECTOR 0xffff
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Reading these registers returns vector mapped to a given event,
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or NO_VECTOR if unmapped. All queue and configuration change
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events are unmapped by default.
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Note that mapping an event to vector might require allocating
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internal device resources, and might fail. Devices report such
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failures by returning the NO_VECTOR value when the relevant
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Vector field is read. After mapping an event to vector, the
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driver must verify success by reading the Vector field value: on
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success, the previously written value is returned, and on
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failure, NO_VECTOR is returned. If a mapping failure is detected,
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the driver can retry mapping with fewervectors, or disable MSI-X.
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Virtqueue Configuration
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As a device can have zero or more virtqueues for bulk data
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transport (for example, the network driver has two), the driver
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needs to configure them as part of the device-specific
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configuration.
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This is done as follows, for each virtqueue a device has:
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Write the virtqueue index (first queue is 0) to the Queue
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Select field.
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Read the virtqueue size from the Queue Size field, which is
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always a power of 2. This controls how big the virtqueue is
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(see below). If this field is 0, the virtqueue does not exist.
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Allocate and zero virtqueue in contiguous physical memory, on a
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4096 byte alignment. Write the physical address, divided by
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4096 to the Queue Address field.[footnote:
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The 4096 is based on the x86 page size, but it's also large
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enough to ensure that the separate parts of the virtqueue are on
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separate cache lines.
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]
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Optionally, if MSI-X capability is present and enabled on the
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device, select a vector to use to request interrupts triggered
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by virtqueue events. Write the MSI-X Table entry number
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corresponding to this vector in Queue Vector field. Read the
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Queue Vector field: on success, previously written value is
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returned; on failure, NO_VECTOR value is returned.
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The Queue Size field controls the total number of bytes required
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for the virtqueue according to the following formula:
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#define ALIGN(x) (((x) + 4095) & ~4095)
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static inline unsigned vring_size(unsigned int qsz)
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{
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return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2
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+ qsz))
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+ ALIGN(sizeof(struct vring_used_elem)*qsz);
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}
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This currently wastes some space with padding, but also allows
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future extensions. The virtqueue layout structure looks like this
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(qsz is the Queue Size field, which is a variable, so this code
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won't compile):
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struct vring {
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/* The actual descriptors (16 bytes each) */
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struct vring_desc desc[qsz];
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/* A ring of available descriptor heads with free-running
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index. */
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struct vring_avail avail;
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// Padding to the next 4096 boundary.
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char pad[];
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// A ring of used descriptor heads with free-running index.
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struct vring_used used;
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};
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A Note on Virtqueue Endianness
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Note that the endian of these fields and everything else in the
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virtqueue is the native endian of the guest, not little-endian as
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PCI normally is. This makes for simpler guest code, and it is
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assumed that the host already has to be deeply aware of the guest
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endian so such an “endian-aware” device is not a significant
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issue.
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Descriptor Table
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The descriptor table refers to the buffers the guest is using for
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the device. The addresses are physical addresses, and the buffers
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can be chained via the next field. Each descriptor describes a
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buffer which is read-only or write-only, but a chain of
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descriptors can contain both read-only and write-only buffers.
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No descriptor chain may be more than 2^32 bytes long in total.struct vring_desc {
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/* Address (guest-physical). */
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u64 addr;
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/* Length. */
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u32 len;
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/* This marks a buffer as continuing via the next field. */
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#define VRING_DESC_F_NEXT 1
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/* This marks a buffer as write-only (otherwise read-only). */
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#define VRING_DESC_F_WRITE 2
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/* This means the buffer contains a list of buffer descriptors.
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*/
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#define VRING_DESC_F_INDIRECT 4
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/* The flags as indicated above. */
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u16 flags;
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/* Next field if flags & NEXT */
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u16 next;
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};
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The number of descriptors in the table is specified by the Queue
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Size field for this virtqueue.
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<sub:Indirect-Descriptors>Indirect Descriptors
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Some devices benefit by concurrently dispatching a large number
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of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be
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used to allow this (see [cha:Reserved-Feature-Bits]). To increase
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ring capacity it is possible to store a table of indirect
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descriptors anywhere in memory, and insert a descriptor in main
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virtqueue (with flags&INDIRECT on) that refers to memory buffer
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containing this indirect descriptor table; fields addr and len
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refer to the indirect table address and length in bytes,
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respectively. The indirect table layout structure looks like this
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(len is the length of the descriptor that refers to this table,
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which is a variable, so this code won't compile):
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struct indirect_descriptor_table {
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/* The actual descriptors (16 bytes each) */
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struct vring_desc desc[len / 16];
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};
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The first indirect descriptor is located at start of the indirect
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descriptor table (index 0), additional indirect descriptors are
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chained by next field. An indirect descriptor without next field
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(with flags&NEXT off) signals the end of the indirect descriptor
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table, and transfers control back to the main virtqueue. An
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indirect descriptor can not refer to another indirect descriptor
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table (flags&INDIRECT must be off). A single indirect descriptor
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table can include both read-only and write-only descriptors;
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write-only flag (flags&WRITE) in the descriptor that refers to it
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is ignored.
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Available Ring
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The available ring refers to what descriptors we are offering the
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device: it refers to the head of a descriptor chain. The “flags”
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field is currently 0 or 1: 1 indicating that we do not need an
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interrupt when the device consumes a descriptor from the
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available ring. Alternatively, the guest can ask the device to
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delay interrupts until an entry with an index specified by the “
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used_event” field is written in the used ring (equivalently,
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until the idx field in the used ring will reach the value
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used_event + 1). The method employed by the device is controlled
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by the VIRTIO_RING_F_EVENT_IDX feature bit (see [cha:Reserved-Feature-Bits]
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). This interrupt suppression is merely an optimization; it may
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not suppress interrupts entirely.
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The “idx” field indicates where we would put the next descriptor
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entry (modulo the ring size). This starts at 0, and increases.
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struct vring_avail {
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#define VRING_AVAIL_F_NO_INTERRUPT 1
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u16 flags;
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u16 idx;
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u16 ring[qsz]; /* qsz is the Queue Size field read from device
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*/
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u16 used_event;
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};
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Used Ring
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The used ring is where the device returns buffers once it is done
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with them. The flags field can be used by the device to hint that
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no notification is necessary when the guest adds to the available
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ring. Alternatively, the “avail_event” field can be used by the
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device to hint that no notification is necessary until an entry
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with an index specified by the “avail_event” is written in the
|
|
available ring (equivalently, until the idx field in the
|
|
available ring will reach the value avail_event + 1). The method
|
|
employed by the device is controlled by the guest through the
|
|
VIRTIO_RING_F_EVENT_IDX feature bit (see [cha:Reserved-Feature-Bits]
|
|
). [footnote:
|
|
These fields are kept here because this is the only part of the
|
|
virtqueue written by the device
|
|
].
|
|
|
|
Each entry in the ring is a pair: the head entry of the
|
|
descriptor chain describing the buffer (this matches an entry
|
|
placed in the available ring by the guest earlier), and the total
|
|
of bytes written into the buffer. The latter is extremely useful
|
|
for guests using untrusted buffers: if you do not know exactly
|
|
how much has been written by the device, you usually have to zero
|
|
the buffer to ensure no data leakage occurs.
|
|
|
|
/* u32 is used here for ids for padding reasons. */
|
|
|
|
struct vring_used_elem {
|
|
|
|
/* Index of start of used descriptor chain. */
|
|
|
|
u32 id;
|
|
|
|
/* Total length of the descriptor chain which was used
|
|
(written to) */
|
|
|
|
u32 len;
|
|
|
|
};
|
|
|
|
|
|
|
|
struct vring_used {
|
|
|
|
#define VRING_USED_F_NO_NOTIFY 1
|
|
|
|
u16 flags;
|
|
|
|
u16 idx;
|
|
|
|
struct vring_used_elem ring[qsz];
|
|
|
|
u16 avail_event;
|
|
|
|
};
|
|
|
|
Helpers for Managing Virtqueues
|
|
|
|
The Linux Kernel Source code contains the definitions above and
|
|
helper routines in a more usable form, in
|
|
include/linux/virtio_ring.h. This was explicitly licensed by IBM
|
|
and Red Hat under the (3-clause) BSD license so that it can be
|
|
freely used by all other projects, and is reproduced (with slight
|
|
variation to remove Linux assumptions) in Appendix A.
|
|
|
|
Device Operation
|
|
|
|
There are two parts to device operation: supplying new buffers to
|
|
the device, and processing used buffers from the device. As an
|
|
example, the virtio network device has two virtqueues: the
|
|
transmit virtqueue and the receive virtqueue. The driver adds
|
|
outgoing (read-only) packets to the transmit virtqueue, and then
|
|
frees them after they are used. Similarly, incoming (write-only)
|
|
buffers are added to the receive virtqueue, and processed after
|
|
they are used.
|
|
|
|
Supplying Buffers to The Device
|
|
|
|
Actual transfer of buffers from the guest OS to the device
|
|
operates as follows:
|
|
|
|
Place the buffer(s) into free descriptor(s).
|
|
|
|
If there are no free descriptors, the guest may choose to
|
|
notify the device even if notifications are suppressed (to
|
|
reduce latency).[footnote:
|
|
The Linux drivers do this only for read-only buffers: for
|
|
write-only buffers, it is assumed that the driver is merely
|
|
trying to keep the receive buffer ring full, and no notification
|
|
of this expected condition is necessary.
|
|
]
|
|
|
|
Place the id of the buffer in the next ring entry of the
|
|
available ring.
|
|
|
|
The steps (1) and (2) may be performed repeatedly if batching
|
|
is possible.
|
|
|
|
A memory barrier should be executed to ensure the device sees
|
|
the updated descriptor table and available ring before the next
|
|
step.
|
|
|
|
The available “idx” field should be increased by the number of
|
|
entries added to the available ring.
|
|
|
|
A memory barrier should be executed to ensure that we update
|
|
the idx field before checking for notification suppression.
|
|
|
|
If notifications are not suppressed, the device should be
|
|
notified of the new buffers.
|
|
|
|
Note that the above code does not take precautions against the
|
|
available ring buffer wrapping around: this is not possible since
|
|
the ring buffer is the same size as the descriptor table, so step
|
|
(1) will prevent such a condition.
|
|
|
|
In addition, the maximum queue size is 32768 (it must be a power
|
|
of 2 which fits in 16 bits), so the 16-bit “idx” value can always
|
|
distinguish between a full and empty buffer.
|
|
|
|
Here is a description of each stage in more detail.
|
|
|
|
Placing Buffers Into The Descriptor Table
|
|
|
|
A buffer consists of zero or more read-only physically-contiguous
|
|
elements followed by zero or more physically-contiguous
|
|
write-only elements (it must have at least one element). This
|
|
algorithm maps it into the descriptor table:
|
|
|
|
for each buffer element, b:
|
|
|
|
Get the next free descriptor table entry, d
|
|
|
|
Set d.addr to the physical address of the start of b
|
|
|
|
Set d.len to the length of b.
|
|
|
|
If b is write-only, set d.flags to VRING_DESC_F_WRITE,
|
|
otherwise 0.
|
|
|
|
If there is a buffer element after this:
|
|
|
|
Set d.next to the index of the next free descriptor element.
|
|
|
|
Set the VRING_DESC_F_NEXT bit in d.flags.
|
|
|
|
In practice, the d.next fields are usually used to chain free
|
|
descriptors, and a separate count kept to check there are enough
|
|
free descriptors before beginning the mappings.
|
|
|
|
Updating The Available Ring
|
|
|
|
The head of the buffer we mapped is the first d in the algorithm
|
|
above. A naive implementation would do the following:
|
|
|
|
avail->ring[avail->idx % qsz] = head;
|
|
|
|
However, in general we can add many descriptors before we update
|
|
the “idx” field (at which point they become visible to the
|
|
device), so we keep a counter of how many we've added:
|
|
|
|
avail->ring[(avail->idx + added++) % qsz] = head;
|
|
|
|
Updating The Index Field
|
|
|
|
Once the idx field of the virtqueue is updated, the device will
|
|
be able to access the descriptor entries we've created and the
|
|
memory they refer to. This is why a memory barrier is generally
|
|
used before the idx update, to ensure it sees the most up-to-date
|
|
copy.
|
|
|
|
The idx field always increments, and we let it wrap naturally at
|
|
65536:
|
|
|
|
avail->idx += added;
|
|
|
|
<sub:Notifying-The-Device>Notifying The Device
|
|
|
|
Device notification occurs by writing the 16-bit virtqueue index
|
|
of this virtqueue to the Queue Notify field of the virtio header
|
|
in the first I/O region of the PCI device. This can be expensive,
|
|
however, so the device can suppress such notifications if it
|
|
doesn't need them. We have to be careful to expose the new idx
|
|
value before checking the suppression flag: it's OK to notify
|
|
gratuitously, but not to omit a required notification. So again,
|
|
we use a memory barrier here before reading the flags or the
|
|
avail_event field.
|
|
|
|
If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if
|
|
the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to
|
|
the PCI configuration space.
|
|
|
|
If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the
|
|
avail_event field in the available ring structure. If the
|
|
available index crossed_the avail_event field value since the
|
|
last notification, we go ahead and write to the PCI configuration
|
|
space. The avail_event field wraps naturally at 65536 as well:
|
|
|
|
(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)
|
|
|
|
<sub:Receiving-Used-Buffers>Receiving Used Buffers From The
|
|
Device
|
|
|
|
Once the device has used a buffer (read from or written to it, or
|
|
parts of both, depending on the nature of the virtqueue and the
|
|
device), it sends an interrupt, following an algorithm very
|
|
similar to the algorithm used for the driver to send the device a
|
|
buffer:
|
|
|
|
Write the head descriptor number to the next field in the used
|
|
ring.
|
|
|
|
Update the used ring idx.
|
|
|
|
Determine whether an interrupt is necessary:
|
|
|
|
If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check
|
|
if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail-
|
|
>flags
|
|
|
|
If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check
|
|
whether the used index crossed the used_event field value
|
|
since the last update. The used_event field wraps naturally
|
|
at 65536 as well:(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)
|
|
|
|
If an interrupt is necessary:
|
|
|
|
If MSI-X capability is disabled:
|
|
|
|
Set the lower bit of the ISR Status field for the device.
|
|
|
|
Send the appropriate PCI interrupt for the device.
|
|
|
|
If MSI-X capability is enabled:
|
|
|
|
Request the appropriate MSI-X interrupt message for the
|
|
device, Queue Vector field sets the MSI-X Table entry
|
|
number.
|
|
|
|
If Queue Vector field value is NO_VECTOR, no interrupt
|
|
message is requested for this event.
|
|
|
|
The guest interrupt handler should:
|
|
|
|
If MSI-X capability is disabled: read the ISR Status field,
|
|
which will reset it to zero. If the lower bit is zero, the
|
|
interrupt was not for this device. Otherwise, the guest driver
|
|
should look through the used rings of each virtqueue for the
|
|
device, to see if any progress has been made by the device
|
|
which requires servicing.
|
|
|
|
If MSI-X capability is enabled: look through the used rings of
|
|
each virtqueue mapped to the specific MSI-X vector for the
|
|
device, to see if any progress has been made by the device
|
|
which requires servicing.
|
|
|
|
For each ring, guest should then disable interrupts by writing
|
|
VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
|
|
It can then process used ring entries finally enabling interrupts
|
|
by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the
|
|
EVENT_IDX field in the available structure, Guest should then
|
|
execute a memory barrier, and then recheck the ring empty
|
|
condition. This is necessary to handle the case where, after the
|
|
last check and before enabling interrupts, an interrupt has been
|
|
suppressed by the device:
|
|
|
|
vring_disable_interrupts(vq);
|
|
|
|
for (;;) {
|
|
|
|
if (vq->last_seen_used != vring->used.idx) {
|
|
|
|
vring_enable_interrupts(vq);
|
|
|
|
mb();
|
|
|
|
if (vq->last_seen_used != vring->used.idx)
|
|
|
|
break;
|
|
|
|
}
|
|
|
|
struct vring_used_elem *e =
|
|
vring.used->ring[vq->last_seen_used%vsz];
|
|
|
|
process_buffer(e);
|
|
|
|
vq->last_seen_used++;
|
|
|
|
}
|
|
|
|
Dealing With Configuration Changes
|
|
|
|
Some virtio PCI devices can change the device configuration
|
|
state, as reflected in the virtio header in the PCI configuration
|
|
space. In this case:
|
|
|
|
If MSI-X capability is disabled: an interrupt is delivered and
|
|
the second highest bit is set in the ISR Status field to
|
|
indicate that the driver should re-examine the configuration
|
|
space.Note that a single interrupt can indicate both that one
|
|
or more virtqueue has been used and that the configuration
|
|
space has changed: even if the config bit is set, virtqueues
|
|
must be scanned.
|
|
|
|
If MSI-X capability is enabled: an interrupt message is
|
|
requested. The Configuration Vector field sets the MSI-X Table
|
|
entry number to use. If Configuration Vector field value is
|
|
NO_VECTOR, no interrupt message is requested for this event.
|
|
|
|
Creating New Device Types
|
|
|
|
Various considerations are necessary when creating a new device
|
|
type:
|
|
|
|
How Many Virtqueues?
|
|
|
|
It is possible that a very simple device will operate entirely
|
|
through its configuration space, but most will need at least one
|
|
virtqueue in which it will place requests. A device with both
|
|
input and output (eg. console and network devices described here)
|
|
need two queues: one which the driver fills with buffers to
|
|
receive input, and one which the driver places buffers to
|
|
transmit output.
|
|
|
|
What Configuration Space Layout?
|
|
|
|
Configuration space is generally used for rarely-changing or
|
|
initialization-time parameters. But it is a limited resource, so
|
|
it might be better to use a virtqueue to update configuration
|
|
information (the network device does this for filtering,
|
|
otherwise the table in the config space could potentially be very
|
|
large).
|
|
|
|
Note that this space is generally the guest's native endian,
|
|
rather than PCI's little-endian.
|
|
|
|
What Device Number?
|
|
|
|
Currently device numbers are assigned quite freely: a simple
|
|
request mail to the author of this document or the Linux
|
|
virtualization mailing list[footnote:
|
|
|
|
https://lists.linux-foundation.org/mailman/listinfo/virtualization
|
|
] will be sufficient to secure a unique one.
|
|
|
|
Meanwhile for experimental drivers, use 65535 and work backwards.
|
|
|
|
How many MSI-X vectors?
|
|
|
|
Using the optional MSI-X capability devices can speed up
|
|
interrupt processing by removing the need to read ISR Status
|
|
register by guest driver (which might be an expensive operation),
|
|
reducing interrupt sharing between devices and queues within the
|
|
device, and handling interrupts from multiple CPUs. However, some
|
|
systems impose a limit (which might be as low as 256) on the
|
|
total number of MSI-X vectors that can be allocated to all
|
|
devices. Devices and/or device drivers should take this into
|
|
account, limiting the number of vectors used unless the device is
|
|
expected to cause a high volume of interrupts. Devices can
|
|
control the number of vectors used by limiting the MSI-X Table
|
|
Size or not presenting MSI-X capability in PCI configuration
|
|
space. Drivers can control this by mapping events to as small
|
|
number of vectors as possible, or disabling MSI-X capability
|
|
altogether.
|
|
|
|
Message Framing
|
|
|
|
The descriptors used for a buffer should not effect the semantics
|
|
of the message, except for the total length of the buffer. For
|
|
example, a network buffer consists of a 10 byte header followed
|
|
by the network packet. Whether this is presented in the ring
|
|
descriptor chain as (say) a 10 byte buffer and a 1514 byte
|
|
buffer, or a single 1524 byte buffer, or even three buffers,
|
|
should have no effect.
|
|
|
|
In particular, no implementation should use the descriptor
|
|
boundaries to determine the size of any header in a request.[footnote:
|
|
The current qemu device implementations mistakenly insist that
|
|
the first descriptor cover the header in these cases exactly, so
|
|
a cautious driver should arrange it so.
|
|
]
|
|
|
|
Device Improvements
|
|
|
|
Any change to configuration space, or new virtqueues, or
|
|
behavioural changes, should be indicated by negotiation of a new
|
|
feature bit. This establishes clarity[footnote:
|
|
Even if it does mean documenting design or implementation
|
|
mistakes!
|
|
] and avoids future expansion problems.
|
|
|
|
Clusters of functionality which are always implemented together
|
|
can use a single bit, but if one feature makes sense without the
|
|
others they should not be gratuitously grouped together to
|
|
conserve feature bits. We can always extend the spec when the
|
|
first person needs more than 24 feature bits for their device.
|
|
|
|
[LaTeX Command: printnomenclature]
|
|
|
|
Appendix A: virtio_ring.h
|
|
|
|
#ifndef VIRTIO_RING_H
|
|
|
|
#define VIRTIO_RING_H
|
|
|
|
/* An interface for efficient virtio implementation.
|
|
|
|
*
|
|
|
|
* This header is BSD licensed so anyone can use the definitions
|
|
|
|
* to implement compatible drivers/servers.
|
|
|
|
*
|
|
|
|
* Copyright 2007, 2009, IBM Corporation
|
|
|
|
* Copyright 2011, Red Hat, Inc
|
|
|
|
* All rights reserved.
|
|
|
|
*
|
|
|
|
* Redistribution and use in source and binary forms, with or
|
|
without
|
|
|
|
* modification, are permitted provided that the following
|
|
conditions
|
|
|
|
* are met:
|
|
|
|
* 1. Redistributions of source code must retain the above
|
|
copyright
|
|
|
|
* notice, this list of conditions and the following
|
|
disclaimer.
|
|
|
|
* 2. Redistributions in binary form must reproduce the above
|
|
copyright
|
|
|
|
* notice, this list of conditions and the following
|
|
disclaimer in the
|
|
|
|
* documentation and/or other materials provided with the
|
|
distribution.
|
|
|
|
* 3. Neither the name of IBM nor the names of its contributors
|
|
|
|
* may be used to endorse or promote products derived from
|
|
this software
|
|
|
|
* without specific prior written permission.
|
|
|
|
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
|
|
CONTRIBUTORS ``AS IS'' AND
|
|
|
|
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
|
|
TO, THE
|
|
|
|
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
|
|
PARTICULAR PURPOSE
|
|
|
|
* ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE
|
|
LIABLE
|
|
|
|
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
|
|
CONSEQUENTIAL
|
|
|
|
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
|
|
SUBSTITUTE GOODS
|
|
|
|
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
|
|
INTERRUPTION)
|
|
|
|
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
|
|
CONTRACT, STRICT
|
|
|
|
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
|
|
IN ANY WAY
|
|
|
|
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
|
|
POSSIBILITY OF
|
|
|
|
* SUCH DAMAGE.
|
|
|
|
*/
|
|
|
|
|
|
|
|
/* This marks a buffer as continuing via the next field. */
|
|
|
|
#define VRING_DESC_F_NEXT 1
|
|
|
|
/* This marks a buffer as write-only (otherwise read-only). */
|
|
|
|
#define VRING_DESC_F_WRITE 2
|
|
|
|
|
|
|
|
/* The Host uses this in used->flags to advise the Guest: don't
|
|
kick me
|
|
|
|
* when you add a buffer. It's unreliable, so it's simply an
|
|
|
|
* optimization. Guest will still kick if it's out of buffers.
|
|
*/
|
|
|
|
#define VRING_USED_F_NO_NOTIFY 1
|
|
|
|
/* The Guest uses this in avail->flags to advise the Host: don't
|
|
|
|
* interrupt me when you consume a buffer. It's unreliable, so
|
|
it's
|
|
|
|
* simply an optimization. */
|
|
|
|
#define VRING_AVAIL_F_NO_INTERRUPT 1
|
|
|
|
|
|
|
|
/* Virtio ring descriptors: 16 bytes.
|
|
|
|
* These can chain together via "next". */
|
|
|
|
struct vring_desc {
|
|
|
|
/* Address (guest-physical). */
|
|
|
|
uint64_t addr;
|
|
|
|
/* Length. */
|
|
|
|
uint32_t len;
|
|
|
|
/* The flags as indicated above. */
|
|
|
|
uint16_t flags;
|
|
|
|
/* We chain unused descriptors via this, too */
|
|
|
|
uint16_t next;
|
|
|
|
};
|
|
|
|
|
|
|
|
struct vring_avail {
|
|
|
|
uint16_t flags;
|
|
|
|
uint16_t idx;
|
|
|
|
uint16_t ring[];
|
|
|
|
uint16_t used_event;
|
|
|
|
};
|
|
|
|
|
|
|
|
/* u32 is used here for ids for padding reasons. */
|
|
|
|
struct vring_used_elem {
|
|
|
|
/* Index of start of used descriptor chain. */
|
|
|
|
uint32_t id;
|
|
|
|
/* Total length of the descriptor chain which was written
|
|
to. */
|
|
|
|
uint32_t len;
|
|
|
|
};
|
|
|
|
|
|
|
|
struct vring_used {
|
|
|
|
uint16_t flags;
|
|
|
|
uint16_t idx;
|
|
|
|
struct vring_used_elem ring[];
|
|
|
|
uint16_t avail_event;
|
|
|
|
};
|
|
|
|
|
|
|
|
struct vring {
|
|
|
|
unsigned int num;
|
|
|
|
|
|
|
|
struct vring_desc *desc;
|
|
|
|
struct vring_avail *avail;
|
|
|
|
struct vring_used *used;
|
|
|
|
};
|
|
|
|
|
|
|
|
/* The standard layout for the ring is a continuous chunk of
|
|
memory which
|
|
|
|
* looks like this. We assume num is a power of 2.
|
|
|
|
*
|
|
|
|
* struct vring {
|
|
|
|
* // The actual descriptors (16 bytes each)
|
|
|
|
* struct vring_desc desc[num];
|
|
|
|
*
|
|
|
|
* // A ring of available descriptor heads with free-running
|
|
index.
|
|
|
|
* __u16 avail_flags;
|
|
|
|
* __u16 avail_idx;
|
|
|
|
* __u16 available[num];
|
|
|
|
*
|
|
|
|
* // Padding to the next align boundary.
|
|
|
|
* char pad[];
|
|
|
|
*
|
|
|
|
* // A ring of used descriptor heads with free-running
|
|
index.
|
|
|
|
* __u16 used_flags;
|
|
|
|
* __u16 EVENT_IDX;
|
|
|
|
* struct vring_used_elem used[num];
|
|
|
|
* };
|
|
|
|
* Note: for virtio PCI, align is 4096.
|
|
|
|
*/
|
|
|
|
static inline void vring_init(struct vring *vr, unsigned int num,
|
|
void *p,
|
|
|
|
unsigned long align)
|
|
|
|
{
|
|
|
|
vr->num = num;
|
|
|
|
vr->desc = p;
|
|
|
|
vr->avail = p + num*sizeof(struct vring_desc);
|
|
|
|
vr->used = (void *)(((unsigned long)&vr->avail->ring[num]
|
|
|
|
+ align-1)
|
|
|
|
& ~(align - 1));
|
|
|
|
}
|
|
|
|
|
|
|
|
static inline unsigned vring_size(unsigned int num, unsigned long
|
|
align)
|
|
|
|
{
|
|
|
|
return ((sizeof(struct vring_desc)*num +
|
|
sizeof(uint16_t)*(2+num)
|
|
|
|
+ align - 1) & ~(align - 1))
|
|
|
|
+ sizeof(uint16_t)*3 + sizeof(struct
|
|
vring_used_elem)*num;
|
|
|
|
}
|
|
|
|
|
|
|
|
static inline int vring_need_event(uint16_t event_idx, uint16_t
|
|
new_idx, uint16_t old_idx)
|
|
|
|
{
|
|
|
|
return (uint16_t)(new_idx - event_idx - 1) <
|
|
(uint16_t)(new_idx - old_idx);
|
|
|
|
}
|
|
|
|
#endif /* VIRTIO_RING_H */
|
|
|
|
<cha:Reserved-Feature-Bits>Appendix B: Reserved Feature Bits
|
|
|
|
Currently there are five device-independent feature bits defined:
|
|
|
|
VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature
|
|
indicates that the driver wants an interrupt if the device runs
|
|
out of available descriptors on a virtqueue, even though
|
|
interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT
|
|
flag or the used_event field. An example of this is the
|
|
networking driver: it doesn't need to know every time a packet
|
|
is transmitted, but it does need to free the transmitted
|
|
packets a finite time after they are transmitted. It can avoid
|
|
using a timer if the device interrupts it when all the packets
|
|
are transmitted.
|
|
|
|
VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature
|
|
indicates that the driver can use descriptors with the
|
|
VRING_DESC_F_INDIRECT flag set, as described in [sub:Indirect-Descriptors]
|
|
.
|
|
|
|
VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event
|
|
and the avail_event fields. If set, it indicates that the
|
|
device should ignore the flags field in the available ring
|
|
structure. Instead, the used_event field in this structure is
|
|
used by guest to suppress device interrupts. Further, the
|
|
driver should ignore the flags field in the used ring
|
|
structure. Instead, the avail_event field in this structure is
|
|
used by the device to suppress notifications. If unset, the
|
|
driver should ignore the used_event field; the device should
|
|
ignore the avail_event field; the flags field is used
|
|
|
|
VIRTIO_F_BAD_FEATURE(30) This feature should never be
|
|
negotiated by the guest; doing so is an indication that the
|
|
guest is faulty[footnote:
|
|
An experimental virtio PCI driver contained in Linux version
|
|
2.6.25 had this problem, and this feature bit can be used to
|
|
detect it.
|
|
]
|
|
|
|
VIRTIO_F_FEATURES_HIGH(31) This feature indicates that the
|
|
device supports feature bits 32:63. If unset, feature bits
|
|
32:63 are unset.
|
|
|
|
Appendix C: Network Device
|
|
|
|
The virtio network device is a virtual ethernet card, and is the
|
|
most complex of the devices supported so far by virtio. It has
|
|
enhanced rapidly and demonstrates clearly how support for new
|
|
features should be added to an existing device. Empty buffers are
|
|
placed in one virtqueue for receiving packets, and outgoing
|
|
packets are enqueued into another for transmission in that order.
|
|
A third command queue is used to control advanced filtering
|
|
features.
|
|
|
|
Configuration
|
|
|
|
Subsystem Device ID 1
|
|
|
|
Virtqueues 0:receiveq. 1:transmitq. 2:controlq[footnote:
|
|
Only if VIRTIO_NET_F_CTRL_VQ set
|
|
]
|
|
|
|
Feature bits
|
|
|
|
VIRTIO_NET_F_CSUM (0) Device handles packets with partial
|
|
checksum
|
|
|
|
VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial
|
|
checksum
|
|
|
|
VIRTIO_NET_F_MAC (5) Device has given MAC address.
|
|
|
|
VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with
|
|
any GSO type.[footnote:
|
|
It was supposed to indicate segmentation offload support, but
|
|
upon further investigation it became clear that multiple bits
|
|
were required.
|
|
]
|
|
|
|
VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.
|
|
|
|
VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.
|
|
|
|
VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.
|
|
|
|
VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.
|
|
|
|
VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.
|
|
|
|
VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.
|
|
|
|
VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.
|
|
|
|
VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.
|
|
|
|
VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.
|
|
|
|
VIRTIO_NET_F_STATUS (16) Configuration status field is
|
|
available.
|
|
|
|
VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.
|
|
|
|
VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.
|
|
|
|
VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.
|
|
|
|
Device configuration layout Two configuration fields are
|
|
currently defined. The mac address field always exists (though
|
|
is only valid if VIRTIO_NET_F_MAC is set), and the status field
|
|
only exists if VIRTIO_NET_F_STATUS is set. Only one bit is
|
|
currently defined for the status field: VIRTIO_NET_S_LINK_UP. #define VIRTIO_NET_S_LINK_UP 1
|
|
|
|
|
|
|
|
struct virtio_net_config {
|
|
|
|
u8 mac[6];
|
|
|
|
u16 status;
|
|
|
|
};
|
|
|
|
Device Initialization
|
|
|
|
The initialization routine should identify the receive and
|
|
transmission virtqueues.
|
|
|
|
If the VIRTIO_NET_F_MAC feature bit is set, the configuration
|
|
space “mac” entry indicates the “physical” address of the the
|
|
network card, otherwise a private MAC address should be
|
|
assigned. All guests are expected to negotiate this feature if
|
|
it is set.
|
|
|
|
If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify
|
|
the control virtqueue.
|
|
|
|
If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link
|
|
status can be read from the bottom bit of the “status” config
|
|
field. Otherwise, the link should be assumed active.
|
|
|
|
The receive virtqueue should be filled with receive buffers.
|
|
This is described in detail below in “Setting Up Receive
|
|
Buffers”.
|
|
|
|
A driver can indicate that it will generate checksumless
|
|
packets by negotating the VIRTIO_NET_F_CSUM feature. This “
|
|
checksum offload” is a common feature on modern network cards.
|
|
|
|
If that feature is negotiated, a driver can use TCP or UDP
|
|
segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4
|
|
(IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and
|
|
VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features. It should
|
|
not send TCP packets requiring segmentation offload which have
|
|
the Explicit Congestion Notification bit set, unless the
|
|
VIRTIO_NET_F_HOST_ECN feature is negotiated.[footnote:
|
|
This is a common restriction in real, older network cards.
|
|
]
|
|
|
|
The converse features are also available: a driver can save the
|
|
virtual device some work by negotiating these features.[footnote:
|
|
For example, a network packet transported between two guests on
|
|
the same system may not require checksumming at all, nor
|
|
segmentation, if both guests are amenable.
|
|
] The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
|
|
checksummed packets can be received, and if it can do that then
|
|
the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
|
|
VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input
|
|
equivalents of the features described above. See “Receiving
|
|
Packets” below.
|
|
|
|
Device Operation
|
|
|
|
Packets are transmitted by placing them in the transmitq, and
|
|
buffers for incoming packets are placed in the receiveq. In each
|
|
case, the packet itself is preceded by a header:
|
|
|
|
struct virtio_net_hdr {
|
|
|
|
#define VIRTIO_NET_HDR_F_NEEDS_CSUM 1
|
|
|
|
u8 flags;
|
|
|
|
#define VIRTIO_NET_HDR_GSO_NONE 0
|
|
|
|
#define VIRTIO_NET_HDR_GSO_TCPV4 1
|
|
|
|
#define VIRTIO_NET_HDR_GSO_UDP 3
|
|
|
|
#define VIRTIO_NET_HDR_GSO_TCPV6 4
|
|
|
|
#define VIRTIO_NET_HDR_GSO_ECN 0x80
|
|
|
|
u8 gso_type;
|
|
|
|
u16 hdr_len;
|
|
|
|
u16 gso_size;
|
|
|
|
u16 csum_start;
|
|
|
|
u16 csum_offset;
|
|
|
|
/* Only if VIRTIO_NET_F_MRG_RXBUF: */
|
|
|
|
u16 num_buffers
|
|
|
|
};
|
|
|
|
The controlq is used to control device features such as
|
|
filtering.
|
|
|
|
Packet Transmission
|
|
|
|
Transmitting a single packet is simple, but varies depending on
|
|
the different features the driver negotiated.
|
|
|
|
If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has
|
|
not been fully checksummed, then the virtio_net_hdr's fields
|
|
are set as follows. Otherwise, the packet must be fully
|
|
checksummed, and flags is zero.
|
|
|
|
flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,
|
|
|
|
<ite:csum_start-is-set>csum_start is set to the offset within
|
|
the packet to begin checksumming, and
|
|
|
|
csum_offset indicates how many bytes after the csum_start the
|
|
new (16 bit ones' complement) checksum should be placed.[footnote:
|
|
For example, consider a partially checksummed TCP (IPv4) packet.
|
|
It will have a 14 byte ethernet header and 20 byte IP header
|
|
followed by the TCP header (with the TCP checksum field 16 bytes
|
|
into that header). csum_start will be 14+20 = 34 (the TCP
|
|
checksum includes the header), and csum_offset will be 16. The
|
|
value in the TCP checksum field will be the sum of the TCP pseudo
|
|
header, so that replacing it by the ones' complement checksum of
|
|
the TCP header and body will give the correct result.
|
|
]
|
|
|
|
<enu:If-the-driver>If the driver negotiated
|
|
VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires
|
|
TCP segmentation or UDP fragmentation, then the “gso_type”
|
|
field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP.
|
|
(Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this
|
|
case, packets larger than 1514 bytes can be transmitted: the
|
|
metadata indicates how to replicate the packet header to cut it
|
|
into smaller packets. The other gso fields are set:
|
|
|
|
hdr_len is a hint to the device as to how much of the header
|
|
needs to be kept to copy into each packet, usually set to the
|
|
length of the headers, including the transport header.[footnote:
|
|
Due to various bugs in implementations, this field is not useful
|
|
as a guarantee of the transport header size.
|
|
]
|
|
|
|
gso_size is the size of the packet beyond that header (ie.
|
|
MSS).
|
|
|
|
If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the
|
|
VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as well,
|
|
indicating that the TCP packet has the ECN bit set.[footnote:
|
|
This case is not handled by some older hardware, so is called out
|
|
specifically in the protocol.
|
|
]
|
|
|
|
If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
|
|
the num_buffers field is set to zero.
|
|
|
|
The header and packet are added as one output buffer to the
|
|
transmitq, and the device is notified of the new entry (see [sub:Notifying-The-Device]
|
|
).[footnote:
|
|
Note that the header will be two bytes longer for the
|
|
VIRTIO_NET_F_MRG_RXBUF case.
|
|
]
|
|
|
|
Packet Transmission Interrupt
|
|
|
|
Often a driver will suppress transmission interrupts using the
|
|
VRING_AVAIL_F_NO_INTERRUPT flag (see [sub:Receiving-Used-Buffers]
|
|
) and check for used packets in the transmit path of following
|
|
packets. However, it will still receive interrupts if the
|
|
VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that
|
|
the transmission queue is completely emptied.
|
|
|
|
The normal behavior in this interrupt handler is to retrieve and
|
|
new descriptors from the used ring and free the corresponding
|
|
headers and packets.
|
|
|
|
Setting Up Receive Buffers
|
|
|
|
It is generally a good idea to keep the receive virtqueue as
|
|
fully populated as possible: if it runs out, network performance
|
|
will suffer.
|
|
|
|
If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or
|
|
VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to
|
|
accept packets of up to 65550 bytes long (the maximum size of a
|
|
TCP or UDP packet, plus the 14 byte ethernet header), otherwise
|
|
1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every
|
|
buffer in the receive queue needs to be at least this length [footnote:
|
|
Obviously each one can be split across multiple descriptor
|
|
elements.
|
|
].
|
|
|
|
If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
|
|
least the size of the struct virtio_net_hdr.
|
|
|
|
Packet Receive Interrupt
|
|
|
|
When a packet is copied into a buffer in the receiveq, the
|
|
optimal path is to disable further interrupts for the receiveq
|
|
(see [sub:Receiving-Used-Buffers]) and process packets until no
|
|
more are found, then re-enable them.
|
|
|
|
Processing packet involves:
|
|
|
|
If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
|
|
then the “num_buffers” field indicates how many descriptors
|
|
this packet is spread over (including this one). This allows
|
|
receipt of large packets without having to allocate large
|
|
buffers. In this case, there will be at least “num_buffers” in
|
|
the used ring, and they should be chained together to form a
|
|
single packet. The other buffers will not begin with a struct
|
|
virtio_net_hdr.
|
|
|
|
If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or
|
|
the “num_buffers” field is one, then the entire packet will be
|
|
contained within this buffer, immediately following the struct
|
|
virtio_net_hdr.
|
|
|
|
If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
|
|
VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be
|
|
set: if so, the checksum on the packet is incomplete and the “
|
|
csum_start” and “csum_offset” fields indicate how to calculate
|
|
it (see [ite:csum_start-is-set]).
|
|
|
|
If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
|
|
negotiated, then the “gso_type” may be something other than
|
|
VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the
|
|
desired MSS (see [enu:If-the-driver]).Control Virtqueue
|
|
|
|
The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is
|
|
negotiated) to send commands to manipulate various features of
|
|
the device which would not easily map into the configuration
|
|
space.
|
|
|
|
All commands are of the following form:
|
|
|
|
struct virtio_net_ctrl {
|
|
|
|
u8 class;
|
|
|
|
u8 command;
|
|
|
|
u8 command-specific-data[];
|
|
|
|
u8 ack;
|
|
|
|
};
|
|
|
|
|
|
|
|
/* ack values */
|
|
|
|
#define VIRTIO_NET_OK 0
|
|
|
|
#define VIRTIO_NET_ERR 1
|
|
|
|
The class, command and command-specific-data are set by the
|
|
driver, and the device sets the ack byte. There is little it can
|
|
do except issue a diagnostic if the ack byte is not
|
|
VIRTIO_NET_OK.
|
|
|
|
Packet Receive Filtering
|
|
|
|
If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can
|
|
send control commands for promiscuous mode, multicast receiving,
|
|
and filtering of MAC addresses.
|
|
|
|
Note that in general, these commands are best-effort: unwanted
|
|
packets may still arrive.
|
|
|
|
Setting Promiscuous Mode
|
|
|
|
#define VIRTIO_NET_CTRL_RX 0
|
|
|
|
#define VIRTIO_NET_CTRL_RX_PROMISC 0
|
|
|
|
#define VIRTIO_NET_CTRL_RX_ALLMULTI 1
|
|
|
|
The class VIRTIO_NET_CTRL_RX has two commands:
|
|
VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and
|
|
VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and
|
|
off. The command-specific-data is one byte containing 0 (off) or
|
|
1 (on).
|
|
|
|
Setting MAC Address Filtering
|
|
|
|
struct virtio_net_ctrl_mac {
|
|
|
|
u32 entries;
|
|
|
|
u8 macs[entries][ETH_ALEN];
|
|
|
|
};
|
|
|
|
|
|
|
|
#define VIRTIO_NET_CTRL_MAC 1
|
|
|
|
#define VIRTIO_NET_CTRL_MAC_TABLE_SET 0
|
|
|
|
The device can filter incoming packets by any number of
|
|
destination MAC addresses.[footnote:
|
|
Since there are no guarantees, it can use a hash filter
|
|
orsilently switch to allmulti or promiscuous mode if it is given
|
|
too many addresses.
|
|
] This table is set using the class VIRTIO_NET_CTRL_MAC and the
|
|
command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data
|
|
is two variable length tables of 6-byte MAC addresses. The first
|
|
table contains unicast addresses, and the second contains
|
|
multicast addresses.
|
|
|
|
VLAN Filtering
|
|
|
|
If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
|
|
can control a VLAN filter table in the device.
|
|
|
|
#define VIRTIO_NET_CTRL_VLAN 2
|
|
|
|
#define VIRTIO_NET_CTRL_VLAN_ADD 0
|
|
|
|
#define VIRTIO_NET_CTRL_VLAN_DEL 1
|
|
|
|
Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL
|
|
command take a 16-bit VLAN id as the command-specific-data.
|
|
|
|
Appendix D: Block Device
|
|
|
|
The virtio block device is a simple virtual block device (ie.
|
|
disk). Read and write requests (and other exotic requests) are
|
|
placed in the queue, and serviced (probably out of order) by the
|
|
device except where noted.
|
|
|
|
Configuration
|
|
|
|
Subsystem Device ID 2
|
|
|
|
Virtqueues 0:requestq.
|
|
|
|
Feature bits
|
|
|
|
VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.
|
|
|
|
VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
|
|
in “size_max”.
|
|
|
|
VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a
|
|
request is in “seg_max”.
|
|
|
|
VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “
|
|
geometry”.
|
|
|
|
VIRTIO_BLK_F_RO (5) Device is read-only.
|
|
|
|
VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.
|
|
|
|
VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
|
|
|
|
VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
|
|
|
|
|
|
|
|
Device configuration layout The capacity of the device
|
|
(expressed in 512-byte sectors) is always present. The
|
|
availability of the others all depend on various feature bits
|
|
as indicated above. struct virtio_blk_config {
|
|
|
|
u64 capacity;
|
|
|
|
u32 size_max;
|
|
|
|
u32 seg_max;
|
|
|
|
struct virtio_blk_geometry {
|
|
|
|
u16 cylinders;
|
|
|
|
u8 heads;
|
|
|
|
u8 sectors;
|
|
|
|
} geometry;
|
|
|
|
u32 blk_size;
|
|
|
|
|
|
|
|
};
|
|
|
|
Device Initialization
|
|
|
|
The device size should be read from the “capacity”
|
|
configuration field. No requests should be submitted which goes
|
|
beyond this limit.
|
|
|
|
If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the
|
|
blk_size field can be read to determine the optimal sector size
|
|
for the driver to use. This does not effect the units used in
|
|
the protocol (always 512 bytes), but awareness of the correct
|
|
value can effect performance.
|
|
|
|
If the VIRTIO_BLK_F_RO feature is set by the device, any write
|
|
requests will fail.
|
|
|
|
|
|
|
|
Device Operation
|
|
|
|
The driver queues requests to the virtqueue, and they are used by
|
|
the device (not necessarily in order). Each request is of form:
|
|
|
|
struct virtio_blk_req {
|
|
|
|
|
|
|
|
u32 type;
|
|
|
|
u32 ioprio;
|
|
|
|
u64 sector;
|
|
|
|
char data[][512];
|
|
|
|
u8 status;
|
|
|
|
};
|
|
|
|
If the device has VIRTIO_BLK_F_SCSI feature, it can also support
|
|
scsi packet command requests, each of these requests is of form:struct virtio_scsi_pc_req {
|
|
|
|
u32 type;
|
|
|
|
u32 ioprio;
|
|
|
|
u64 sector;
|
|
|
|
char cmd[];
|
|
|
|
char data[][512];
|
|
|
|
#define SCSI_SENSE_BUFFERSIZE 96
|
|
|
|
u8 sense[SCSI_SENSE_BUFFERSIZE];
|
|
|
|
u32 errors;
|
|
|
|
u32 data_len;
|
|
|
|
u32 sense_len;
|
|
|
|
u32 residual;
|
|
|
|
u8 status;
|
|
|
|
};
|
|
|
|
The type of the request is either a read (VIRTIO_BLK_T_IN), a
|
|
write (VIRTIO_BLK_T_OUT), a scsi packet command
|
|
(VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT[footnote:
|
|
the SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device
|
|
does not distinguish between them
|
|
]) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT[footnote:
|
|
the FLUSH and FLUSH_OUT types are equivalent, the device does not
|
|
distinguish between them
|
|
]). If the device has VIRTIO_BLK_F_BARRIER feature the high bit
|
|
(VIRTIO_BLK_T_BARRIER) indicates that this request acts as a
|
|
barrier and that all preceding requests must be complete before
|
|
this one, and all following requests must not be started until
|
|
this is complete. Note that a barrier does not flush caches in
|
|
the underlying backend device in host, and thus does not serve as
|
|
data consistency guarantee. Driver must use FLUSH request to
|
|
flush the host cache.
|
|
|
|
#define VIRTIO_BLK_T_IN 0
|
|
|
|
#define VIRTIO_BLK_T_OUT 1
|
|
|
|
#define VIRTIO_BLK_T_SCSI_CMD 2
|
|
|
|
#define VIRTIO_BLK_T_SCSI_CMD_OUT 3
|
|
|
|
#define VIRTIO_BLK_T_FLUSH 4
|
|
|
|
#define VIRTIO_BLK_T_FLUSH_OUT 5
|
|
|
|
#define VIRTIO_BLK_T_BARRIER 0x80000000
|
|
|
|
The ioprio field is a hint about the relative priorities of
|
|
requests to the device: higher numbers indicate more important
|
|
requests.
|
|
|
|
The sector number indicates the offset (multiplied by 512) where
|
|
the read or write is to occur. This field is unused and set to 0
|
|
for scsi packet commands and for flush commands.
|
|
|
|
The cmd field is only present for scsi packet command requests,
|
|
and indicates the command to perform. This field must reside in a
|
|
single, separate read-only buffer; command length can be derived
|
|
from the length of this buffer.
|
|
|
|
Note that these first three (four for scsi packet commands)
|
|
fields are always read-only: the data field is either read-only
|
|
or write-only, depending on the request. The size of the read or
|
|
write can be derived from the total size of the request buffers.
|
|
|
|
The sense field is only present for scsi packet command requests,
|
|
and indicates the buffer for scsi sense data.
|
|
|
|
The data_len field is only present for scsi packet command
|
|
requests, this field is deprecated, and should be ignored by the
|
|
driver. Historically, devices copied data length there.
|
|
|
|
The sense_len field is only present for scsi packet command
|
|
requests and indicates the number of bytes actually written to
|
|
the sense buffer.
|
|
|
|
The residual field is only present for scsi packet command
|
|
requests and indicates the residual size, calculated as data
|
|
length - number of bytes actually transferred.
|
|
|
|
The final status byte is written by the device: either
|
|
VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest
|
|
error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:#define VIRTIO_BLK_S_OK 0
|
|
|
|
#define VIRTIO_BLK_S_IOERR 1
|
|
|
|
#define VIRTIO_BLK_S_UNSUPP 2
|
|
|
|
Historically, devices assumed that the fields type, ioprio and
|
|
sector reside in a single, separate read-only buffer; the fields
|
|
errors, data_len, sense_len and residual reside in a single,
|
|
separate write-only buffer; the sense field in a separate
|
|
write-only buffer of size 96 bytes, by itself; the fields errors,
|
|
data_len, sense_len and residual in a single write-only buffer;
|
|
and the status field is a separate read-only buffer of size 1
|
|
byte, by itself.
|
|
|
|
Appendix E: Console Device
|
|
|
|
The virtio console device is a simple device for data input and
|
|
output. A device may have one or more ports. Each port has a pair
|
|
of input and output virtqueues. Moreover, a device has a pair of
|
|
control IO virtqueues. The control virtqueues are used to
|
|
communicate information between the device and the driver about
|
|
ports being opened and closed on either side of the connection,
|
|
indication from the host about whether a particular port is a
|
|
console port, adding new ports, port hot-plug/unplug, etc., and
|
|
indication from the guest about whether a port or a device was
|
|
successfully added, port open/close, etc.. For data IO, one or
|
|
more empty buffers are placed in the receive queue for incoming
|
|
data and outgoing characters are placed in the transmit queue.
|
|
|
|
Configuration
|
|
|
|
Subsystem Device ID 3
|
|
|
|
Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control
|
|
receiveq[footnote:
|
|
Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set
|
|
], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1),
|
|
...
|
|
|
|
Feature bits
|
|
|
|
VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields
|
|
are valid.
|
|
|
|
VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple
|
|
ports; configuration fields nr_ports and max_nr_ports are
|
|
valid and control virtqueues will be used.
|
|
|
|
Device configuration layout The size of the console is supplied
|
|
in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
|
|
is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
|
|
is set, the maximum number of ports supported by the device can
|
|
be fetched.struct virtio_console_config {
|
|
|
|
u16 cols;
|
|
|
|
u16 rows;
|
|
|
|
|
|
|
|
u32 max_nr_ports;
|
|
|
|
};
|
|
|
|
Device Initialization
|
|
|
|
If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
|
|
can read the console dimensions from the configuration fields.
|
|
|
|
If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the
|
|
driver can spawn multiple ports, not all of which may be
|
|
attached to a console. Some could be generic ports. In this
|
|
case, the control virtqueues are enabled and according to the
|
|
max_nr_ports configuration-space value, the appropriate number
|
|
of virtqueues are created. A control message indicating the
|
|
driver is ready is sent to the host. The host can then send
|
|
control messages for adding new ports to the device. After
|
|
creating and initializing each port, a
|
|
VIRTIO_CONSOLE_PORT_READY control message is sent to the host
|
|
for that port so the host can let us know of any additional
|
|
configuration options set for that port.
|
|
|
|
The receiveq for each port is populated with one or more
|
|
receive buffers.
|
|
|
|
Device Operation
|
|
|
|
For output, a buffer containing the characters is placed in the
|
|
port's transmitq.[footnote:
|
|
Because this is high importance and low bandwidth, the current
|
|
Linux implementation polls for the buffer to be used, rather than
|
|
waiting for an interrupt, simplifying the implementation
|
|
significantly. However, for generic serial ports with the
|
|
O_NONBLOCK flag set, the polling limitation is relaxed and the
|
|
consumed buffers are freed upon the next write or poll call or
|
|
when a port is closed or hot-unplugged.
|
|
]
|
|
|
|
When a buffer is used in the receiveq (signalled by an
|
|
interrupt), the contents is the input to the port associated
|
|
with the virtqueue for which the notification was received.
|
|
|
|
If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a
|
|
configuration change interrupt may occur. The updated size can
|
|
be read from the configuration fields.
|
|
|
|
If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT
|
|
feature, active ports are announced by the host using the
|
|
VIRTIO_CONSOLE_PORT_ADD control message. The same message is
|
|
used for port hot-plug as well.
|
|
|
|
If the host specified a port `name', a sysfs attribute is
|
|
created with the name filled in, so that udev rules can be
|
|
written that can create a symlink from the port's name to the
|
|
char device for port discovery by applications in the guest.
|
|
|
|
Changes to ports' state are effected by control messages.
|
|
Appropriate action is taken on the port indicated in the
|
|
control message. The layout of the structure of the control
|
|
buffer and the events associated are:struct virtio_console_control {
|
|
|
|
uint32_t id; /* Port number */
|
|
|
|
uint16_t event; /* The kind of control event */
|
|
|
|
uint16_t value; /* Extra information for the event */
|
|
|
|
};
|
|
|
|
|
|
|
|
/* Some events for the internal messages (control packets) */
|
|
|
|
|
|
|
|
#define VIRTIO_CONSOLE_DEVICE_READY 0
|
|
|
|
#define VIRTIO_CONSOLE_PORT_ADD 1
|
|
|
|
#define VIRTIO_CONSOLE_PORT_REMOVE 2
|
|
|
|
#define VIRTIO_CONSOLE_PORT_READY 3
|
|
|
|
#define VIRTIO_CONSOLE_CONSOLE_PORT 4
|
|
|
|
#define VIRTIO_CONSOLE_RESIZE 5
|
|
|
|
#define VIRTIO_CONSOLE_PORT_OPEN 6
|
|
|
|
#define VIRTIO_CONSOLE_PORT_NAME 7
|
|
|
|
Appendix F: Entropy Device
|
|
|
|
The virtio entropy device supplies high-quality randomness for
|
|
guest use.
|
|
|
|
Configuration
|
|
|
|
Subsystem Device ID 4
|
|
|
|
Virtqueues 0:requestq.
|
|
|
|
Feature bits None currently defined
|
|
|
|
Device configuration layout None currently defined.
|
|
|
|
Device Initialization
|
|
|
|
The virtqueue is initialized
|
|
|
|
Device Operation
|
|
|
|
When the driver requires random bytes, it places the descriptor
|
|
of one or more buffers in the queue. It will be completely filled
|
|
by random data by the device.
|
|
|
|
Appendix G: Memory Balloon Device
|
|
|
|
The virtio memory balloon device is a primitive device for
|
|
managing guest memory: the device asks for a certain amount of
|
|
memory, and the guest supplies it (or withdraws it, if the device
|
|
has more than it asks for). This allows the guest to adapt to
|
|
changes in allowance of underlying physical memory. If the
|
|
feature is negotiated, the device can also be used to communicate
|
|
guest memory statistics to the host.
|
|
|
|
Configuration
|
|
|
|
Subsystem Device ID 5
|
|
|
|
Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[footnote:
|
|
Only if VIRTIO_BALLON_F_STATS_VQ set
|
|
]
|
|
|
|
Feature bits
|
|
|
|
VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before
|
|
pages from the balloon are used.
|
|
|
|
VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest
|
|
memory statistics is present.
|
|
|
|
Device configuration layout Both fields of this configuration
|
|
are always available. Note that they are little endian, despite
|
|
convention that device fields are guest endian:struct virtio_balloon_config {
|
|
|
|
u32 num_pages;
|
|
|
|
u32 actual;
|
|
|
|
};
|
|
|
|
Device Initialization
|
|
|
|
The inflate and deflate virtqueues are identified.
|
|
|
|
If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
|
|
|
|
Identify the stats virtqueue.
|
|
|
|
Add one empty buffer to the stats virtqueue and notify the
|
|
host.
|
|
|
|
Device operation begins immediately.
|
|
|
|
Device Operation
|
|
|
|
Memory Ballooning The device is driven by the receipt of a
|
|
configuration change interrupt.
|
|
|
|
The “num_pages” configuration field is examined. If this is
|
|
greater than the “actual” number of pages, memory must be given
|
|
to the balloon. If it is less than the “actual” number of
|
|
pages, memory may be taken back from the balloon for general
|
|
use.
|
|
|
|
To supply memory to the balloon (aka. inflate):
|
|
|
|
The driver constructs an array of addresses of unused memory
|
|
pages. These addresses are divided by 4096[footnote:
|
|
This is historical, and independent of the guest page size
|
|
] and the descriptor describing the resulting 32-bit array is
|
|
added to the inflateq.
|
|
|
|
To remove memory from the balloon (aka. deflate):
|
|
|
|
The driver constructs an array of addresses of memory pages it
|
|
has previously given to the balloon, as described above. This
|
|
descriptor is added to the deflateq.
|
|
|
|
If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the
|
|
guest may not use these requested pages until that descriptor
|
|
in the deflateq has been used by the device.
|
|
|
|
Otherwise, the guest may begin to re-use pages previously given
|
|
to the balloon before the device has acknowledged their
|
|
withdrawal. [footnote:
|
|
In this case, deflation advice is merely a courtesy
|
|
]
|
|
|
|
In either case, once the device has completed the inflation or
|
|
deflation, the “actual” field of the configuration should be
|
|
updated to reflect the new number of pages in the balloon.[footnote:
|
|
As updates to configuration space are not atomic, this field
|
|
isn't particularly reliable, but can be used to diagnose buggy
|
|
guests.
|
|
]
|
|
|
|
Memory Statistics
|
|
|
|
The stats virtqueue is atypical because communication is driven
|
|
by the device (not the driver). The channel becomes active at
|
|
driver initialization time when the driver adds an empty buffer
|
|
and notifies the device. A request for memory statistics proceeds
|
|
as follows:
|
|
|
|
The device pushes the buffer onto the used ring and sends an
|
|
interrupt.
|
|
|
|
The driver pops the used buffer and discards it.
|
|
|
|
The driver collects memory statistics and writes them into a
|
|
new buffer.
|
|
|
|
The driver adds the buffer to the virtqueue and notifies the
|
|
device.
|
|
|
|
The device pops the buffer (retaining it to initiate a
|
|
subsequent request) and consumes the statistics.
|
|
|
|
Memory Statistics Format Each statistic consists of a 16 bit
|
|
tag and a 64 bit value. Both quantities are represented in the
|
|
native endian of the guest. All statistics are optional and the
|
|
driver may choose which ones to supply. To guarantee backwards
|
|
compatibility, unsupported statistics should be omitted.
|
|
|
|
struct virtio_balloon_stat {
|
|
|
|
#define VIRTIO_BALLOON_S_SWAP_IN 0
|
|
|
|
#define VIRTIO_BALLOON_S_SWAP_OUT 1
|
|
|
|
#define VIRTIO_BALLOON_S_MAJFLT 2
|
|
|
|
#define VIRTIO_BALLOON_S_MINFLT 3
|
|
|
|
#define VIRTIO_BALLOON_S_MEMFREE 4
|
|
|
|
#define VIRTIO_BALLOON_S_MEMTOT 5
|
|
|
|
u16 tag;
|
|
|
|
u64 val;
|
|
|
|
} __attribute__((packed));
|
|
|
|
Tags
|
|
|
|
VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been
|
|
swapped in (in bytes).
|
|
|
|
VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been
|
|
swapped out to disk (in bytes).
|
|
|
|
VIRTIO_BALLOON_S_MAJFLT The number of major page faults that
|
|
have occurred.
|
|
|
|
VIRTIO_BALLOON_S_MINFLT The number of minor page faults that
|
|
have occurred.
|
|
|
|
VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used
|
|
for any purpose (in bytes).
|
|
|
|
VIRTIO_BALLOON_S_MEMTOT The total amount of memory available
|
|
(in bytes).
|
|
|