224 lines
9.5 KiB
ReStructuredText
224 lines
9.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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=======
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IOMMUFD
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=======
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:Author: Jason Gunthorpe
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:Author: Kevin Tian
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Overview
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========
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IOMMUFD is the user API to control the IOMMU subsystem as it relates to managing
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IO page tables from userspace using file descriptors. It intends to be general
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and consumable by any driver that wants to expose DMA to userspace. These
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drivers are eventually expected to deprecate any internal IOMMU logic
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they may already/historically implement (e.g. vfio_iommu_type1.c).
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At minimum iommufd provides universal support of managing I/O address spaces and
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I/O page tables for all IOMMUs, with room in the design to add non-generic
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features to cater to specific hardware functionality.
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In this context the capital letter (IOMMUFD) refers to the subsystem while the
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small letter (iommufd) refers to the file descriptors created via /dev/iommu for
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use by userspace.
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Key Concepts
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============
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User Visible Objects
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--------------------
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Following IOMMUFD objects are exposed to userspace:
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- IOMMUFD_OBJ_IOAS, representing an I/O address space (IOAS), allowing map/unmap
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of user space memory into ranges of I/O Virtual Address (IOVA).
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The IOAS is a functional replacement for the VFIO container, and like the VFIO
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container it copies an IOVA map to a list of iommu_domains held within it.
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- IOMMUFD_OBJ_DEVICE, representing a device that is bound to iommufd by an
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external driver.
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- IOMMUFD_OBJ_HW_PAGETABLE, representing an actual hardware I/O page table
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(i.e. a single struct iommu_domain) managed by the iommu driver.
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The IOAS has a list of HW_PAGETABLES that share the same IOVA mapping and
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it will synchronize its mapping with each member HW_PAGETABLE.
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All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
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The diagram below shows relationship between user-visible objects and kernel
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datastructures (external to iommufd), with numbers referred to operations
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creating the objects and links::
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_________________________________________________________
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| iommufd |
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| [1] |
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| _________________ |
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| | | [3] [2] |
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| | | ____________ __________ |
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| | IOAS |<--| |<------| | |
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| | | |HW_PAGETABLE| | DEVICE | |
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| | | |____________| |__________| |
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| |_________________| | | |
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|_________|___________________|___________________|_______|
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| _____v______ _______v_____
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| PFN storage | | | |
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|------------>|iommu_domain| |struct device|
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|____________| |_____________|
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1. IOMMUFD_OBJ_IOAS is created via the IOMMU_IOAS_ALLOC uAPI. An iommufd can
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hold multiple IOAS objects. IOAS is the most generic object and does not
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expose interfaces that are specific to single IOMMU drivers. All operations
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on the IOAS must operate equally on each of the iommu_domains inside of it.
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2. IOMMUFD_OBJ_DEVICE is created when an external driver calls the IOMMUFD kAPI
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to bind a device to an iommufd. The driver is expected to implement a set of
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ioctls to allow userspace to initiate the binding operation. Successful
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completion of this operation establishes the desired DMA ownership over the
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device. The driver must also set the driver_managed_dma flag and must not
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touch the device until this operation succeeds.
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3. IOMMUFD_OBJ_HW_PAGETABLE is created when an external driver calls the IOMMUFD
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kAPI to attach a bound device to an IOAS. Similarly the external driver uAPI
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allows userspace to initiate the attaching operation. If a compatible
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pagetable already exists then it is reused for the attachment. Otherwise a
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new pagetable object and iommu_domain is created. Successful completion of
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this operation sets up the linkages among IOAS, device and iommu_domain. Once
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this completes the device could do DMA.
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Every iommu_domain inside the IOAS is also represented to userspace as a
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HW_PAGETABLE object.
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.. note::
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Future IOMMUFD updates will provide an API to create and manipulate the
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HW_PAGETABLE directly.
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A device can only bind to an iommufd due to DMA ownership claim and attach to at
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most one IOAS object (no support of PASID yet).
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Kernel Datastructure
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--------------------
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User visible objects are backed by following datastructures:
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- iommufd_ioas for IOMMUFD_OBJ_IOAS.
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- iommufd_device for IOMMUFD_OBJ_DEVICE.
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- iommufd_hw_pagetable for IOMMUFD_OBJ_HW_PAGETABLE.
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Several terminologies when looking at these datastructures:
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- Automatic domain - refers to an iommu domain created automatically when
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attaching a device to an IOAS object. This is compatible to the semantics of
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VFIO type1.
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- Manual domain - refers to an iommu domain designated by the user as the
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target pagetable to be attached to by a device. Though currently there are
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no uAPIs to directly create such domain, the datastructure and algorithms
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are ready for handling that use case.
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- In-kernel user - refers to something like a VFIO mdev that is using the
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IOMMUFD access interface to access the IOAS. This starts by creating an
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iommufd_access object that is similar to the domain binding a physical device
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would do. The access object will then allow converting IOVA ranges into struct
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page * lists, or doing direct read/write to an IOVA.
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iommufd_ioas serves as the metadata datastructure to manage how IOVA ranges are
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mapped to memory pages, composed of:
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- struct io_pagetable holding the IOVA map
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- struct iopt_area's representing populated portions of IOVA
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- struct iopt_pages representing the storage of PFNs
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- struct iommu_domain representing the IO page table in the IOMMU
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- struct iopt_pages_access representing in-kernel users of PFNs
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- struct xarray pinned_pfns holding a list of pages pinned by in-kernel users
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Each iopt_pages represents a logical linear array of full PFNs. The PFNs are
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ultimately derived from userspace VAs via an mm_struct. Once they have been
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pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pfns
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xarray if they have been pinned through an iommufd_access.
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PFN have to be copied between all combinations of storage locations, depending
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on what domains are present and what kinds of in-kernel "software access" users
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exist. The mechanism ensures that a page is pinned only once.
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An io_pagetable is composed of iopt_areas pointing at iopt_pages, along with a
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list of iommu_domains that mirror the IOVA to PFN map.
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Multiple io_pagetable-s, through their iopt_area-s, can share a single
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iopt_pages which avoids multi-pinning and double accounting of page
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consumption.
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iommufd_ioas is sharable between subsystems, e.g. VFIO and VDPA, as long as
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devices managed by different subsystems are bound to a same iommufd.
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IOMMUFD User API
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================
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.. kernel-doc:: include/uapi/linux/iommufd.h
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IOMMUFD Kernel API
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==================
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The IOMMUFD kAPI is device-centric with group-related tricks managed behind the
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scene. This allows the external drivers calling such kAPI to implement a simple
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device-centric uAPI for connecting its device to an iommufd, instead of
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explicitly imposing the group semantics in its uAPI as VFIO does.
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.. kernel-doc:: drivers/iommu/iommufd/device.c
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:export:
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.. kernel-doc:: drivers/iommu/iommufd/main.c
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:export:
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VFIO and IOMMUFD
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----------------
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Connecting a VFIO device to iommufd can be done in two ways.
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First is a VFIO compatible way by directly implementing the /dev/vfio/vfio
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container IOCTLs by mapping them into io_pagetable operations. Doing so allows
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the use of iommufd in legacy VFIO applications by symlinking /dev/vfio/vfio to
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/dev/iommufd or extending VFIO to SET_CONTAINER using an iommufd instead of a
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container fd.
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The second approach directly extends VFIO to support a new set of device-centric
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user API based on aforementioned IOMMUFD kernel API. It requires userspace
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change but better matches the IOMMUFD API semantics and easier to support new
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iommufd features when comparing it to the first approach.
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Currently both approaches are still work-in-progress.
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There are still a few gaps to be resolved to catch up with VFIO type1, as
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documented in iommufd_vfio_check_extension().
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Future TODOs
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============
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Currently IOMMUFD supports only kernel-managed I/O page table, similar to VFIO
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type1. New features on the radar include:
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- Binding iommu_domain's to PASID/SSID
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- Userspace page tables, for ARM, x86 and S390
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- Kernel bypass'd invalidation of user page tables
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- Re-use of the KVM page table in the IOMMU
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- Dirty page tracking in the IOMMU
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- Runtime Increase/Decrease of IOPTE size
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- PRI support with faults resolved in userspace
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