docs: iommu: Remove outdated Documentation/userspace-api/iommu.rst
The Documentation/userspace-api/iommu.rst file has become outdated due to the removal of associated structures and APIs. Specifically, struct such as iommu_cache_invalidate_info and guest pasid related uapi were removed in commit0c9f178778("iommu: Remove guest pasid related interfaces and definitions"). And the corresponding uapi/linux/iommu.h file was removed in commit00a9bc6070("iommu: Move iommu fault data to linux/iommu.h"). Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com> Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com> Link: https://lore.kernel.org/r/20240702120617.26882-1-mark-pk.tsai@mediatek.com [will: Remove stale reference to 'iommu' from index.rst] Signed-off-by: Will Deacon <will@kernel.org>
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Mark-PK Tsai
Will Deacon
parent
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commit
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@ -44,7 +44,6 @@ Devices and I/O
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accelerators/ocxl
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dma-buf-alloc-exchange
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gpio/index
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iommu
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iommufd
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media/index
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dcdbas
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@ -1,209 +0,0 @@
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.. SPDX-License-Identifier: GPL-2.0
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.. iommu:
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=====================================
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IOMMU Userspace API
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=====================================
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IOMMU UAPI is used for virtualization cases where communications are
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needed between physical and virtual IOMMU drivers. For baremetal
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usage, the IOMMU is a system device which does not need to communicate
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with userspace directly.
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The primary use cases are guest Shared Virtual Address (SVA) and
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guest IO virtual address (IOVA), wherein the vIOMMU implementation
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relies on the physical IOMMU and for this reason requires interactions
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with the host driver.
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.. contents:: :local:
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Functionalities
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===============
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Communications of user and kernel involve both directions. The
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supported user-kernel APIs are as follows:
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1. Bind/Unbind guest PASID (e.g. Intel VT-d)
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2. Bind/Unbind guest PASID table (e.g. ARM SMMU)
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3. Invalidate IOMMU caches upon guest requests
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4. Report errors to the guest and serve page requests
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Requirements
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============
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The IOMMU UAPIs are generic and extensible to meet the following
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requirements:
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1. Emulated and para-virtualised vIOMMUs
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2. Multiple vendors (Intel VT-d, ARM SMMU, etc.)
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3. Extensions to the UAPI shall not break existing userspace
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Interfaces
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==========
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Although the data structures defined in IOMMU UAPI are self-contained,
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there are no user API functions introduced. Instead, IOMMU UAPI is
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designed to work with existing user driver frameworks such as VFIO.
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Extension Rules & Precautions
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-----------------------------
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When IOMMU UAPI gets extended, the data structures can *only* be
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modified in two ways:
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1. Adding new fields by re-purposing the padding[] field. No size change.
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2. Adding new union members at the end. May increase the structure sizes.
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No new fields can be added *after* the variable sized union in that it
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will break backward compatibility when offset moves. A new flag must
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be introduced whenever a change affects the structure using either
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method. The IOMMU driver processes the data based on flags which
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ensures backward compatibility.
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Version field is only reserved for the unlikely event of UAPI upgrade
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at its entirety.
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It's *always* the caller's responsibility to indicate the size of the
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structure passed by setting argsz appropriately.
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Though at the same time, argsz is user provided data which is not
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trusted. The argsz field allows the user app to indicate how much data
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it is providing; it's still the kernel's responsibility to validate
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whether it's correct and sufficient for the requested operation.
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Compatibility Checking
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----------------------
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When IOMMU UAPI extension results in some structure size increase,
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IOMMU UAPI code shall handle the following cases:
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1. User and kernel has exact size match
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2. An older user with older kernel header (smaller UAPI size) running on a
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newer kernel (larger UAPI size)
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3. A newer user with newer kernel header (larger UAPI size) running
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on an older kernel.
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4. A malicious/misbehaving user passing illegal/invalid size but within
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range. The data may contain garbage.
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Feature Checking
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----------------
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While launching a guest with vIOMMU, it is strongly advised to check
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the compatibility upfront, as some subsequent errors happening during
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vIOMMU operation, such as cache invalidation failures cannot be nicely
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escalated to the guest due to IOMMU specifications. This can lead to
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catastrophic failures for the users.
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User applications such as QEMU are expected to import kernel UAPI
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headers. Backward compatibility is supported per feature flags.
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For example, an older QEMU (with older kernel header) can run on newer
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kernel. Newer QEMU (with new kernel header) may refuse to initialize
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on an older kernel if new feature flags are not supported by older
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kernel. Simply recompiling existing code with newer kernel header should
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not be an issue in that only existing flags are used.
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IOMMU vendor driver should report the below features to IOMMU UAPI
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consumers (e.g. via VFIO).
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1. IOMMU_NESTING_FEAT_SYSWIDE_PASID
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2. IOMMU_NESTING_FEAT_BIND_PGTBL
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3. IOMMU_NESTING_FEAT_BIND_PASID_TABLE
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4. IOMMU_NESTING_FEAT_CACHE_INVLD
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5. IOMMU_NESTING_FEAT_PAGE_REQUEST
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Take VFIO as example, upon request from VFIO userspace (e.g. QEMU),
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VFIO kernel code shall query IOMMU vendor driver for the support of
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the above features. Query result can then be reported back to the
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userspace caller. Details can be found in
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Documentation/driver-api/vfio.rst.
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Data Passing Example with VFIO
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------------------------------
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As the ubiquitous userspace driver framework, VFIO is already IOMMU
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aware and shares many key concepts such as device model, group, and
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protection domain. Other user driver frameworks can also be extended
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to support IOMMU UAPI but it is outside the scope of this document.
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In this tight-knit VFIO-IOMMU interface, the ultimate consumer of the
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IOMMU UAPI data is the host IOMMU driver. VFIO facilitates user-kernel
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transport, capability checking, security, and life cycle management of
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process address space ID (PASID).
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VFIO layer conveys the data structures down to the IOMMU driver. It
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follows the pattern below::
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struct {
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__u32 argsz;
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__u32 flags;
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__u8 data[];
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};
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Here data[] contains the IOMMU UAPI data structures. VFIO has the
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freedom to bundle the data as well as parse data size based on its own flags.
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In order to determine the size and feature set of the user data, argsz
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and flags (or the equivalent) are also embedded in the IOMMU UAPI data
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structures.
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A "__u32 argsz" field is *always* at the beginning of each structure.
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For example:
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::
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struct iommu_cache_invalidate_info {
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__u32 argsz;
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#define IOMMU_CACHE_INVALIDATE_INFO_VERSION_1 1
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__u32 version;
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/* IOMMU paging structure cache */
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#define IOMMU_CACHE_INV_TYPE_IOTLB (1 << 0) /* IOMMU IOTLB */
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#define IOMMU_CACHE_INV_TYPE_DEV_IOTLB (1 << 1) /* Device IOTLB */
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#define IOMMU_CACHE_INV_TYPE_PASID (1 << 2) /* PASID cache */
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#define IOMMU_CACHE_INV_TYPE_NR (3)
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__u8 cache;
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__u8 granularity;
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__u8 padding[6];
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union {
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struct iommu_inv_pasid_info pasid_info;
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struct iommu_inv_addr_info addr_info;
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} granu;
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};
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VFIO is responsible for checking its own argsz and flags. It then
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invokes appropriate IOMMU UAPI functions. The user pointers are passed
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to the IOMMU layer for further processing. The responsibilities are
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divided as follows:
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- Generic IOMMU layer checks argsz range based on UAPI data in the
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current kernel version.
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- Generic IOMMU layer checks content of the UAPI data for non-zero
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reserved bits in flags, padding fields, and unsupported version.
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This is to ensure not breaking userspace in the future when these
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fields or flags are used.
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- Vendor IOMMU driver checks argsz based on vendor flags. UAPI data
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is consumed based on flags. Vendor driver has access to
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unadulterated argsz value in case of vendor specific future
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extensions. Currently, it does not perform the copy_from_user()
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itself. A __user pointer can be provided in some future scenarios
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where there's vendor data outside of the structure definition.
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IOMMU code treats UAPI data in two categories:
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- structure contains vendor data
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(Example: iommu_uapi_cache_invalidate())
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- structure contains only generic data
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(Example: iommu_uapi_sva_bind_gpasid())
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Sharing UAPI with in-kernel users
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---------------------------------
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For UAPIs that are shared with in-kernel users, a wrapper function is
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provided to distinguish the callers. For example,
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Userspace caller ::
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int iommu_uapi_sva_unbind_gpasid(struct iommu_domain *domain,
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struct device *dev,
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void __user *udata)
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In-kernel caller ::
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int iommu_sva_unbind_gpasid(struct iommu_domain *domain,
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struct device *dev, ioasid_t ioasid);
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@ -11543,7 +11543,6 @@ L: iommu@lists.linux.dev
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S: Maintained
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T: git git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git
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F: Documentation/devicetree/bindings/iommu/
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F: Documentation/userspace-api/iommu.rst
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F: drivers/iommu/
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F: include/linux/iommu.h
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F: include/linux/iova.h
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