42 lines
2.2 KiB
Plaintext
42 lines
2.2 KiB
Plaintext
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Hollis Blanchard <hollisb@us.ibm.com>
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15 Apr 2008
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Various notes on the implementation of KVM for PowerPC 440:
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To enforce isolation, host userspace, guest kernel, and guest userspace all
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run at user privilege level. Only the host kernel runs in supervisor mode.
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Executing privileged instructions in the guest traps into KVM (in the host
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kernel), where we decode and emulate them. Through this technique, unmodified
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440 Linux kernels can be run (slowly) as guests. Future performance work will
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focus on reducing the overhead and frequency of these traps.
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The usual code flow is started from userspace invoking an "run" ioctl, which
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causes KVM to switch into guest context. We use IVPR to hijack the host
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interrupt vectors while running the guest, which allows us to direct all
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interrupts to kvmppc_handle_interrupt(). At this point, we could either
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- handle the interrupt completely (e.g. emulate "mtspr SPRG0"), or
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- let the host interrupt handler run (e.g. when the decrementer fires), or
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- return to host userspace (e.g. when the guest performs device MMIO)
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Address spaces: We take advantage of the fact that Linux doesn't use the AS=1
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address space (in host or guest), which gives us virtual address space to use
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for guest mappings. While the guest is running, the host kernel remains mapped
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in AS=0, but the guest can only use AS=1 mappings.
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TLB entries: The TLB entries covering the host linear mapping remain
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present while running the guest. This reduces the overhead of lightweight
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exits, which are handled by KVM running in the host kernel. We keep three
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copies of the TLB:
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- guest TLB: contents of the TLB as the guest sees it
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- shadow TLB: the TLB that is actually in hardware while guest is running
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- host TLB: to restore TLB state when context switching guest -> host
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When a TLB miss occurs because a mapping was not present in the shadow TLB,
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but was present in the guest TLB, KVM handles the fault without invoking the
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guest. Large guest pages are backed by multiple 4KB shadow pages through this
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mechanism.
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IO: MMIO and DCR accesses are emulated by userspace. We use virtio for network
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and block IO, so those drivers must be enabled in the guest. It's possible
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that some qemu device emulation (e.g. e1000 or rtl8139) may also work with
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little effort.
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