mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-23 17:30:02 +00:00
![Randy Dunlap](/assets/img/avatar_default.png)
Correct spelling problems for Documentation/userspace-api/ as reported by codespell. Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: linux-doc@vger.kernel.org Cc: Sean Young <sean@mess.org> Cc: Mauro Carvalho Chehab <mchehab@kernel.org> Cc: linux-media@vger.kernel.org Link: https://lore.kernel.org/r/20230129231053.20863-10-rdunlap@infradead.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
223 lines
9.5 KiB
ReStructuredText
223 lines
9.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
|
|
|
|
=======
|
|
IOMMUFD
|
|
=======
|
|
|
|
:Author: Jason Gunthorpe
|
|
:Author: Kevin Tian
|
|
|
|
Overview
|
|
========
|
|
|
|
IOMMUFD is the user API to control the IOMMU subsystem as it relates to managing
|
|
IO page tables from userspace using file descriptors. It intends to be general
|
|
and consumable by any driver that wants to expose DMA to userspace. These
|
|
drivers are eventually expected to deprecate any internal IOMMU logic
|
|
they may already/historically implement (e.g. vfio_iommu_type1.c).
|
|
|
|
At minimum iommufd provides universal support of managing I/O address spaces and
|
|
I/O page tables for all IOMMUs, with room in the design to add non-generic
|
|
features to cater to specific hardware functionality.
|
|
|
|
In this context the capital letter (IOMMUFD) refers to the subsystem while the
|
|
small letter (iommufd) refers to the file descriptors created via /dev/iommu for
|
|
use by userspace.
|
|
|
|
Key Concepts
|
|
============
|
|
|
|
User Visible Objects
|
|
--------------------
|
|
|
|
Following IOMMUFD objects are exposed to userspace:
|
|
|
|
- IOMMUFD_OBJ_IOAS, representing an I/O address space (IOAS), allowing map/unmap
|
|
of user space memory into ranges of I/O Virtual Address (IOVA).
|
|
|
|
The IOAS is a functional replacement for the VFIO container, and like the VFIO
|
|
container it copies an IOVA map to a list of iommu_domains held within it.
|
|
|
|
- IOMMUFD_OBJ_DEVICE, representing a device that is bound to iommufd by an
|
|
external driver.
|
|
|
|
- IOMMUFD_OBJ_HW_PAGETABLE, representing an actual hardware I/O page table
|
|
(i.e. a single struct iommu_domain) managed by the iommu driver.
|
|
|
|
The IOAS has a list of HW_PAGETABLES that share the same IOVA mapping and
|
|
it will synchronize its mapping with each member HW_PAGETABLE.
|
|
|
|
All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
|
|
|
|
The diagram below shows relationship between user-visible objects and kernel
|
|
datastructures (external to iommufd), with numbers referred to operations
|
|
creating the objects and links::
|
|
|
|
_________________________________________________________
|
|
| iommufd |
|
|
| [1] |
|
|
| _________________ |
|
|
| | | |
|
|
| | | |
|
|
| | | |
|
|
| | | |
|
|
| | | |
|
|
| | | |
|
|
| | | [3] [2] |
|
|
| | | ____________ __________ |
|
|
| | IOAS |<--| |<------| | |
|
|
| | | |HW_PAGETABLE| | DEVICE | |
|
|
| | | |____________| |__________| |
|
|
| | | | | |
|
|
| | | | | |
|
|
| | | | | |
|
|
| | | | | |
|
|
| | | | | |
|
|
| |_________________| | | |
|
|
| | | | |
|
|
|_________|___________________|___________________|_______|
|
|
| | |
|
|
| _____v______ _______v_____
|
|
| PFN storage | | | |
|
|
|------------>|iommu_domain| |struct device|
|
|
|____________| |_____________|
|
|
|
|
1. IOMMUFD_OBJ_IOAS is created via the IOMMU_IOAS_ALLOC uAPI. An iommufd can
|
|
hold multiple IOAS objects. IOAS is the most generic object and does not
|
|
expose interfaces that are specific to single IOMMU drivers. All operations
|
|
on the IOAS must operate equally on each of the iommu_domains inside of it.
|
|
|
|
2. IOMMUFD_OBJ_DEVICE is created when an external driver calls the IOMMUFD kAPI
|
|
to bind a device to an iommufd. The driver is expected to implement a set of
|
|
ioctls to allow userspace to initiate the binding operation. Successful
|
|
completion of this operation establishes the desired DMA ownership over the
|
|
device. The driver must also set the driver_managed_dma flag and must not
|
|
touch the device until this operation succeeds.
|
|
|
|
3. IOMMUFD_OBJ_HW_PAGETABLE is created when an external driver calls the IOMMUFD
|
|
kAPI to attach a bound device to an IOAS. Similarly the external driver uAPI
|
|
allows userspace to initiate the attaching operation. If a compatible
|
|
pagetable already exists then it is reused for the attachment. Otherwise a
|
|
new pagetable object and iommu_domain is created. Successful completion of
|
|
this operation sets up the linkages among IOAS, device and iommu_domain. Once
|
|
this completes the device could do DMA.
|
|
|
|
Every iommu_domain inside the IOAS is also represented to userspace as a
|
|
HW_PAGETABLE object.
|
|
|
|
.. note::
|
|
|
|
Future IOMMUFD updates will provide an API to create and manipulate the
|
|
HW_PAGETABLE directly.
|
|
|
|
A device can only bind to an iommufd due to DMA ownership claim and attach to at
|
|
most one IOAS object (no support of PASID yet).
|
|
|
|
Kernel Datastructure
|
|
--------------------
|
|
|
|
User visible objects are backed by following datastructures:
|
|
|
|
- iommufd_ioas for IOMMUFD_OBJ_IOAS.
|
|
- iommufd_device for IOMMUFD_OBJ_DEVICE.
|
|
- iommufd_hw_pagetable for IOMMUFD_OBJ_HW_PAGETABLE.
|
|
|
|
Several terminologies when looking at these datastructures:
|
|
|
|
- Automatic domain - refers to an iommu domain created automatically when
|
|
attaching a device to an IOAS object. This is compatible to the semantics of
|
|
VFIO type1.
|
|
|
|
- Manual domain - refers to an iommu domain designated by the user as the
|
|
target pagetable to be attached to by a device. Though currently there are
|
|
no uAPIs to directly create such domain, the datastructure and algorithms
|
|
are ready for handling that use case.
|
|
|
|
- In-kernel user - refers to something like a VFIO mdev that is using the
|
|
IOMMUFD access interface to access the IOAS. This starts by creating an
|
|
iommufd_access object that is similar to the domain binding a physical device
|
|
would do. The access object will then allow converting IOVA ranges into struct
|
|
page * lists, or doing direct read/write to an IOVA.
|
|
|
|
iommufd_ioas serves as the metadata datastructure to manage how IOVA ranges are
|
|
mapped to memory pages, composed of:
|
|
|
|
- struct io_pagetable holding the IOVA map
|
|
- struct iopt_area's representing populated portions of IOVA
|
|
- struct iopt_pages representing the storage of PFNs
|
|
- struct iommu_domain representing the IO page table in the IOMMU
|
|
- struct iopt_pages_access representing in-kernel users of PFNs
|
|
- struct xarray pinned_pfns holding a list of pages pinned by in-kernel users
|
|
|
|
Each iopt_pages represents a logical linear array of full PFNs. The PFNs are
|
|
ultimately derived from userspace VAs via an mm_struct. Once they have been
|
|
pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pfns
|
|
xarray if they have been pinned through an iommufd_access.
|
|
|
|
PFN have to be copied between all combinations of storage locations, depending
|
|
on what domains are present and what kinds of in-kernel "software access" users
|
|
exist. The mechanism ensures that a page is pinned only once.
|
|
|
|
An io_pagetable is composed of iopt_areas pointing at iopt_pages, along with a
|
|
list of iommu_domains that mirror the IOVA to PFN map.
|
|
|
|
Multiple io_pagetable-s, through their iopt_area-s, can share a single
|
|
iopt_pages which avoids multi-pinning and double accounting of page
|
|
consumption.
|
|
|
|
iommufd_ioas is shareable between subsystems, e.g. VFIO and VDPA, as long as
|
|
devices managed by different subsystems are bound to a same iommufd.
|
|
|
|
IOMMUFD User API
|
|
================
|
|
|
|
.. kernel-doc:: include/uapi/linux/iommufd.h
|
|
|
|
IOMMUFD Kernel API
|
|
==================
|
|
|
|
The IOMMUFD kAPI is device-centric with group-related tricks managed behind the
|
|
scene. This allows the external drivers calling such kAPI to implement a simple
|
|
device-centric uAPI for connecting its device to an iommufd, instead of
|
|
explicitly imposing the group semantics in its uAPI as VFIO does.
|
|
|
|
.. kernel-doc:: drivers/iommu/iommufd/device.c
|
|
:export:
|
|
|
|
.. kernel-doc:: drivers/iommu/iommufd/main.c
|
|
:export:
|
|
|
|
VFIO and IOMMUFD
|
|
----------------
|
|
|
|
Connecting a VFIO device to iommufd can be done in two ways.
|
|
|
|
First is a VFIO compatible way by directly implementing the /dev/vfio/vfio
|
|
container IOCTLs by mapping them into io_pagetable operations. Doing so allows
|
|
the use of iommufd in legacy VFIO applications by symlinking /dev/vfio/vfio to
|
|
/dev/iommufd or extending VFIO to SET_CONTAINER using an iommufd instead of a
|
|
container fd.
|
|
|
|
The second approach directly extends VFIO to support a new set of device-centric
|
|
user API based on aforementioned IOMMUFD kernel API. It requires userspace
|
|
change but better matches the IOMMUFD API semantics and easier to support new
|
|
iommufd features when comparing it to the first approach.
|
|
|
|
Currently both approaches are still work-in-progress.
|
|
|
|
There are still a few gaps to be resolved to catch up with VFIO type1, as
|
|
documented in iommufd_vfio_check_extension().
|
|
|
|
Future TODOs
|
|
============
|
|
|
|
Currently IOMMUFD supports only kernel-managed I/O page table, similar to VFIO
|
|
type1. New features on the radar include:
|
|
|
|
- Binding iommu_domain's to PASID/SSID
|
|
- Userspace page tables, for ARM, x86 and S390
|
|
- Kernel bypass'd invalidation of user page tables
|
|
- Re-use of the KVM page table in the IOMMU
|
|
- Dirty page tracking in the IOMMU
|
|
- Runtime Increase/Decrease of IOPTE size
|
|
- PRI support with faults resolved in userspace
|