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diff --git a/Documentation/userspace-api/iommu.rst b/Documentation/userspace-api/iommu.rst new file mode 100644 index 000000000..d3108c151 --- /dev/null +++ b/Documentation/userspace-api/iommu.rst @@ -0,0 +1,209 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. iommu: + +===================================== +IOMMU Userspace API +===================================== + +IOMMU UAPI is used for virtualization cases where communications are +needed between physical and virtual IOMMU drivers. For baremetal +usage, the IOMMU is a system device which does not need to communicate +with userspace directly. + +The primary use cases are guest Shared Virtual Address (SVA) and +guest IO virtual address (IOVA), wherein the vIOMMU implementation +relies on the physical IOMMU and for this reason requires interactions +with the host driver. + +.. contents:: :local: + +Functionalities +=============== +Communications of user and kernel involve both directions. The +supported user-kernel APIs are as follows: + +1. Bind/Unbind guest PASID (e.g. Intel VT-d) +2. Bind/Unbind guest PASID table (e.g. ARM SMMU) +3. Invalidate IOMMU caches upon guest requests +4. Report errors to the guest and serve page requests + +Requirements +============ +The IOMMU UAPIs are generic and extensible to meet the following +requirements: + +1. Emulated and para-virtualised vIOMMUs +2. Multiple vendors (Intel VT-d, ARM SMMU, etc.) +3. Extensions to the UAPI shall not break existing userspace + +Interfaces +========== +Although the data structures defined in IOMMU UAPI are self-contained, +there are no user API functions introduced. Instead, IOMMU UAPI is +designed to work with existing user driver frameworks such as VFIO. + +Extension Rules & Precautions +----------------------------- +When IOMMU UAPI gets extended, the data structures can *only* be +modified in two ways: + +1. Adding new fields by re-purposing the padding[] field. No size change. +2. Adding new union members at the end. May increase the structure sizes. + +No new fields can be added *after* the variable sized union in that it +will break backward compatibility when offset moves. A new flag must +be introduced whenever a change affects the structure using either +method. The IOMMU driver processes the data based on flags which +ensures backward compatibility. + +Version field is only reserved for the unlikely event of UAPI upgrade +at its entirety. + +It's *always* the caller's responsibility to indicate the size of the +structure passed by setting argsz appropriately. +Though at the same time, argsz is user provided data which is not +trusted. The argsz field allows the user app to indicate how much data +it is providing; it's still the kernel's responsibility to validate +whether it's correct and sufficient for the requested operation. + +Compatibility Checking +---------------------- +When IOMMU UAPI extension results in some structure size increase, +IOMMU UAPI code shall handle the following cases: + +1. User and kernel has exact size match +2. An older user with older kernel header (smaller UAPI size) running on a + newer kernel (larger UAPI size) +3. A newer user with newer kernel header (larger UAPI size) running + on an older kernel. +4. A malicious/misbehaving user passing illegal/invalid size but within + range. The data may contain garbage. + +Feature Checking +---------------- +While launching a guest with vIOMMU, it is strongly advised to check +the compatibility upfront, as some subsequent errors happening during +vIOMMU operation, such as cache invalidation failures cannot be nicely +escalated to the guest due to IOMMU specifications. This can lead to +catastrophic failures for the users. + +User applications such as QEMU are expected to import kernel UAPI +headers. Backward compatibility is supported per feature flags. +For example, an older QEMU (with older kernel header) can run on newer +kernel. Newer QEMU (with new kernel header) may refuse to initialize +on an older kernel if new feature flags are not supported by older +kernel. Simply recompiling existing code with newer kernel header should +not be an issue in that only existing flags are used. + +IOMMU vendor driver should report the below features to IOMMU UAPI +consumers (e.g. via VFIO). + +1. IOMMU_NESTING_FEAT_SYSWIDE_PASID +2. IOMMU_NESTING_FEAT_BIND_PGTBL +3. IOMMU_NESTING_FEAT_BIND_PASID_TABLE +4. IOMMU_NESTING_FEAT_CACHE_INVLD +5. IOMMU_NESTING_FEAT_PAGE_REQUEST + +Take VFIO as example, upon request from VFIO userspace (e.g. QEMU), +VFIO kernel code shall query IOMMU vendor driver for the support of +the above features. Query result can then be reported back to the +userspace caller. Details can be found in +Documentation/driver-api/vfio.rst. + + +Data Passing Example with VFIO +------------------------------ +As the ubiquitous userspace driver framework, VFIO is already IOMMU +aware and shares many key concepts such as device model, group, and +protection domain. Other user driver frameworks can also be extended +to support IOMMU UAPI but it is outside the scope of this document. + +In this tight-knit VFIO-IOMMU interface, the ultimate consumer of the +IOMMU UAPI data is the host IOMMU driver. VFIO facilitates user-kernel +transport, capability checking, security, and life cycle management of +process address space ID (PASID). + +VFIO layer conveys the data structures down to the IOMMU driver. It +follows the pattern below:: + + struct { + __u32 argsz; + __u32 flags; + __u8 data[]; + }; + +Here data[] contains the IOMMU UAPI data structures. VFIO has the +freedom to bundle the data as well as parse data size based on its own flags. + +In order to determine the size and feature set of the user data, argsz +and flags (or the equivalent) are also embedded in the IOMMU UAPI data +structures. + +A "__u32 argsz" field is *always* at the beginning of each structure. + +For example: +:: + + struct iommu_cache_invalidate_info { + __u32 argsz; + #define IOMMU_CACHE_INVALIDATE_INFO_VERSION_1 1 + __u32 version; + /* IOMMU paging structure cache */ + #define IOMMU_CACHE_INV_TYPE_IOTLB (1 << 0) /* IOMMU IOTLB */ + #define IOMMU_CACHE_INV_TYPE_DEV_IOTLB (1 << 1) /* Device IOTLB */ + #define IOMMU_CACHE_INV_TYPE_PASID (1 << 2) /* PASID cache */ + #define IOMMU_CACHE_INV_TYPE_NR (3) + __u8 cache; + __u8 granularity; + __u8 padding[6]; + union { + struct iommu_inv_pasid_info pasid_info; + struct iommu_inv_addr_info addr_info; + } granu; + }; + +VFIO is responsible for checking its own argsz and flags. It then +invokes appropriate IOMMU UAPI functions. The user pointers are passed +to the IOMMU layer for further processing. The responsibilities are +divided as follows: + +- Generic IOMMU layer checks argsz range based on UAPI data in the + current kernel version. + +- Generic IOMMU layer checks content of the UAPI data for non-zero + reserved bits in flags, padding fields, and unsupported version. + This is to ensure not breaking userspace in the future when these + fields or flags are used. + +- Vendor IOMMU driver checks argsz based on vendor flags. UAPI data + is consumed based on flags. Vendor driver has access to + unadulterated argsz value in case of vendor specific future + extensions. Currently, it does not perform the copy_from_user() + itself. A __user pointer can be provided in some future scenarios + where there's vendor data outside of the structure definition. + +IOMMU code treats UAPI data in two categories: + +- structure contains vendor data + (Example: iommu_uapi_cache_invalidate()) + +- structure contains only generic data + (Example: iommu_uapi_sva_bind_gpasid()) + + + +Sharing UAPI with in-kernel users +--------------------------------- +For UAPIs that are shared with in-kernel users, a wrapper function is +provided to distinguish the callers. For example, + +Userspace caller :: + + int iommu_uapi_sva_unbind_gpasid(struct iommu_domain *domain, + struct device *dev, + void __user *udata) + +In-kernel caller :: + + int iommu_sva_unbind_gpasid(struct iommu_domain *domain, + struct device *dev, ioasid_t ioasid); |