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iommufd: Document overview of iommufd

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Add iommufd into the documentation tree, and supply initial documentation.
Much of this is linked from code comments by kdoc.

Link: https://lore.kernel.org/r/5-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
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Documentation/userspace-api/index.rst
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ebpf/index
ioctl/index
iommu
iommufd
media/index
netlink/index
sysfs-platform_profile

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.. 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 sharable 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