ARK-OS/arkos/kernel/Documentation/userspace-api/iommufd.rst

.. 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_HWPT_PAGING, representing an actual hardware I/O page table
  (i.e. a single struct iommu_domain) managed by the iommu driver. "PAGING"
  primarily indicates this type of HWPT should be linked to an IOAS. It also
  indicates that it is backed by an iommu_domain with __IOMMU_DOMAIN_PAGING
  feature flag. This can be either an UNMANAGED stage-1 domain for a device
  running in the user space, or a nesting parent stage-2 domain for mappings
  from guest-level physical addresses to host-level physical addresses.

  The IOAS has a list of HWPT_PAGINGs that share the same IOVA mapping and
  it will synchronize its mapping with each member HWPT_PAGING.

- IOMMUFD_OBJ_HWPT_NESTED, representing an actual hardware I/O page table
  (i.e. a single struct iommu_domain) managed by user space (e.g. guest OS).
  "NESTED" indicates that this type of HWPT should be linked to an HWPT_PAGING.
  It also indicates that it is backed by an iommu_domain that has a type of
  IOMMU_DOMAIN_NESTED. This must be a stage-1 domain for a device running in
  the user space (e.g. in a guest VM enabling the IOMMU nested translation
  feature.) As such, it must be created with a given nesting parent stage-2
  domain to associate to. This nested stage-1 page table managed by the user
  space usually has mappings from guest-level I/O virtual addresses to guest-
  level physical addresses.

- IOMMUFD_FAULT, representing a software queue for an HWPT reporting IO page
  faults using the IOMMU HW's PRI (Page Request Interface). This queue object
  provides user space an FD to poll the page fault events and also to respond
  to those events. A FAULT object must be created first to get a fault_id that
  could be then used to allocate a fault-enabled HWPT via the IOMMU_HWPT_ALLOC
  command by setting the IOMMU_HWPT_FAULT_ID_VALID bit in its flags field.

- IOMMUFD_OBJ_VIOMMU, representing a slice of the physical IOMMU instance,
  passed to or shared with a VM. It may be some HW-accelerated virtualization
  features and some SW resources used by the VM. For examples:

  * Security namespace for guest owned ID, e.g. guest-controlled cache tags
  * Non-device-affiliated event reporting, e.g. invalidation queue errors
  * Access to a shareable nesting parent pagetable across physical IOMMUs
  * Virtualization of various platforms IDs, e.g. RIDs and others
  * Delivery of paravirtualized invalidation
  * Direct assigned invalidation queues
  * Direct assigned interrupts

  Such a vIOMMU object generally has the access to a nesting parent pagetable
  to support some HW-accelerated virtualization features. So, a vIOMMU object
  must be created given a nesting parent HWPT_PAGING object, and then it would
  encapsulate that HWPT_PAGING object. Therefore, a vIOMMU object can be used
  to allocate an HWPT_NESTED object in place of the encapsulated HWPT_PAGING.

  .. note::

     The name "vIOMMU" isn't necessarily identical to a virtualized IOMMU in a
     VM. A VM can have one giant virtualized IOMMU running on a machine having
     multiple physical IOMMUs, in which case the VMM will dispatch the requests
     or configurations from this single virtualized IOMMU instance to multiple
     vIOMMU objects created for individual slices of different physical IOMMUs.
     In other words, a vIOMMU object is always a representation of one physical
     IOMMU, not necessarily of a virtualized IOMMU. For VMMs that want the full
     virtualization features from physical IOMMUs, it is suggested to build the
     same number of virtualized IOMMUs as the number of physical IOMMUs, so the
     passed-through devices would be connected to their own virtualized IOMMUs
     backed by corresponding vIOMMU objects, in which case a guest OS would do
     the "dispatch" naturally instead of VMM trappings.

- IOMMUFD_OBJ_VDEVICE, representing a virtual device for an IOMMUFD_OBJ_DEVICE
  against an IOMMUFD_OBJ_VIOMMU. This virtual device holds the device's virtual
  information or attributes (related to the vIOMMU) in a VM. An immediate vDATA
  example can be the virtual ID of the device on a vIOMMU, which is a unique ID
  that VMM assigns to the device for a translation channel/port of the vIOMMU,
  e.g. vSID of ARM SMMUv3, vDeviceID of AMD IOMMU, and vRID of Intel VT-d to a
  Context Table. Potential use cases of some advanced security information can
  be forwarded via this object too, such as security level or realm information
  in a Confidential Compute Architecture. A VMM should create a vDEVICE object
  to forward all the device information in a VM, when it connects a device to a
  vIOMMU, which is a separate ioctl call from attaching the same device to an
  HWPT_PAGING that the vIOMMU holds.

- IOMMUFD_OBJ_VEVENTQ, representing a software queue for a vIOMMU to report its
  events such as translation faults occurred to a nested stage-1 (excluding I/O
  page faults that should go through IOMMUFD_OBJ_FAULT) and HW-specific events.
  This queue object provides user space an FD to poll/read the vIOMMU events. A
  vIOMMU object must be created first to get its viommu_id, which could be then
  used to allocate a vEVENTQ. Each vIOMMU can support multiple types of vEVENTS,
  but is confined to one vEVENTQ per vEVENTQ type.

- IOMMUFD_OBJ_HW_QUEUE, representing a hardware accelerated queue, as a subset
  of IOMMU's virtualization features, for the IOMMU HW to directly read or write
  the virtual queue memory owned by a guest OS. This HW-acceleration feature can
  allow VM to work with the IOMMU HW directly without a VM Exit, so as to reduce
  overhead from the hypercalls. Along with the HW QUEUE object, iommufd provides
  user space an mmap interface for VMM to mmap a physical MMIO region from the
  host physical address space to the guest physical address space, allowing the
  guest OS to directly control the allocated HW QUEUE. Thus, when allocating a
  HW QUEUE, the VMM must request a pair of mmap info (offset/length) and pass in
  exactly to an mmap syscall via its offset and length arguments.

All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.

The diagrams below show relationships between user-visible objects and kernel
datastructures (external to iommufd), with numbers referred to operations
creating the objects and links::

  _______________________________________________________________________
 |                      iommufd (HWPT_PAGING only)                       |
 |                                                                       |
 |        [1]                  [3]                                [2]    |
 |  ________________      _____________                        ________  |
 | |                |    |             |                      |        | |
 | |      IOAS      |<---| HWPT_PAGING |<---------------------| DEVICE | |
 | |________________|    |_____________|                      |________| |
 |         |                    |                                  |     |
 |_________|____________________|__________________________________|_____|
           |                    |                                  |
           |              ______v_____                          ___v__
           | PFN storage |  (paging)  |                        |struct|
           |------------>|iommu_domain|<-----------------------|device|
                         |____________|                        |______|

  _______________________________________________________________________
 |                      iommufd (with HWPT_NESTED)                       |
 |                                                                       |
 |        [1]                  [3]                [4]             [2]    |
 |  ________________      _____________      _____________     ________  |
 | |                |    |             |    |             |   |        | |
 | |      IOAS      |<---| HWPT_PAGING |<---| HWPT_NESTED |<--| DEVICE | |
 | |________________|    |_____________|    |_____________|   |________| |
 |         |                    |                  |               |     |
 |_________|____________________|__________________|_______________|_____|
           |                    |                  |               |
           |              ______v_____       ______v_____       ___v__
           | PFN storage |  (paging)  |     |  (nested)  |     |struct|
           |------------>|iommu_domain|<----|iommu_domain|<----|device|
                         |____________|     |____________|     |______|

  _______________________________________________________________________
 |                      iommufd (with vIOMMU/vDEVICE)                    |
 |                                                                       |
 |                             [5]                [6]                    |
 |                        _____________      _____________               |
 |                       |             |    |             |              |
 |      |----------------|    vIOMMU   |<---|   vDEVICE   |<----|        |
 |      |                |             |    |_____________|     |        |
 |      |                |             |                        |        |
 |      |      [1]       |             |          [4]           | [2]    |
 |      |     ______     |             |     _____________     _|______  |
 |      |    |      |    |     [3]     |    |             |   |        | |
 |      |    | IOAS |<---|(HWPT_PAGING)|<---| HWPT_NESTED |<--| DEVICE | |
 |      |    |______|    |_____________|    |_____________|   |________| |
 |      |        |              |                  |               |     |
 |______|________|______________|__________________|_______________|_____|
        |        |              |                  |               |
  ______v_____   |        ______v_____       ______v_____       ___v__
 |   struct   |  |  PFN  |  (paging)  |     |  (nested)  |     |struct|
 |iommu_device|  |------>|iommu_domain|<----|iommu_domain|<----|device|
 |____________|   storage|____________|     |____________|     |______|

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_HWPT_PAGING can be created in two ways:

   * IOMMUFD_OBJ_HWPT_PAGING is automatically 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 member HWPT_PAGING object exists in the IOAS's HWPT_PAGING
     list, then it will be reused. Otherwise a new HWPT_PAGING that represents
     an iommu_domain to userspace will be created, and then added to the list.
     Successful completion of this operation sets up the linkages among IOAS,
     device and iommu_domain. Once this completes the device could do DMA.

   * IOMMUFD_OBJ_HWPT_PAGING can be manually created via the IOMMU_HWPT_ALLOC
     uAPI, provided an ioas_id via @pt_id to associate the new HWPT_PAGING to
     the corresponding IOAS object. The benefit of this manual allocation is to
     allow allocation flags (defined in enum iommufd_hwpt_alloc_flags), e.g. it
     allocates a nesting parent HWPT_PAGING if the IOMMU_HWPT_ALLOC_NEST_PARENT
     flag is set.

4. IOMMUFD_OBJ_HWPT_NESTED can be only manually created via the IOMMU_HWPT_ALLOC
   uAPI, provided an hwpt_id or a viommu_id of a vIOMMU object encapsulating a
   nesting parent HWPT_PAGING via @pt_id to associate the new HWPT_NESTED object
   to the corresponding HWPT_PAGING object. The associating HWPT_PAGING object
   must be a nesting parent manually allocated via the same uAPI previously with
   an IOMMU_HWPT_ALLOC_NEST_PARENT flag, otherwise the allocation will fail. The
   allocation will be further validated by the IOMMU driver to ensure that the
   nesting parent domain and the nested domain being allocated are compatible.
   Successful completion of this operation sets up linkages among IOAS, device,
   and iommu_domains. Once this completes the device could do DMA via a 2-stage
   translation, a.k.a nested translation. Note that multiple HWPT_NESTED objects
   can be allocated by (and then associated to) the same nesting parent.

   .. note::

      Either a manual IOMMUFD_OBJ_HWPT_PAGING or an IOMMUFD_OBJ_HWPT_NESTED is
      created via the same IOMMU_HWPT_ALLOC uAPI. The difference is at the type
      of the object passed in via the @pt_id field of struct iommufd_hwpt_alloc.

5. IOMMUFD_OBJ_VIOMMU can be only manually created via the IOMMU_VIOMMU_ALLOC
   uAPI, provided a dev_id (for the device's physical IOMMU to back the vIOMMU)
   and an hwpt_id (to associate the vIOMMU to a nesting parent HWPT_PAGING). The
   iommufd core will link the vIOMMU object to the struct iommu_device that the
   struct device is behind. And an IOMMU driver can implement a viommu_alloc op
   to allocate its own vIOMMU data structure embedding the core-level structure
   iommufd_viommu and some driver-specific data. If necessary, the driver can
   also configure its HW virtualization feature for that vIOMMU (and thus for
   the VM). Successful completion of this operation sets up the linkages between
   the vIOMMU object and the HWPT_PAGING, then this vIOMMU object can be used
   as a nesting parent object to allocate an HWPT_NESTED object described above.

6. IOMMUFD_OBJ_VDEVICE can be only manually created via the IOMMU_VDEVICE_ALLOC
   uAPI, provided a viommu_id for an iommufd_viommu object and a dev_id for an
   iommufd_device object. The vDEVICE object will be the binding between these
   two parent objects. Another @virt_id will be also set via the uAPI providing
   the iommufd core an index to store the vDEVICE object to a vDEVICE array per
   vIOMMU. If necessary, the IOMMU driver may choose to implement a vdevce_alloc
   op to init its HW for virtualization feature related to a vDEVICE. Successful
   completion of this operation sets up the linkages between vIOMMU and device.

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_hwpt_paging for IOMMUFD_OBJ_HWPT_PAGING.
- iommufd_hwpt_nested for IOMMUFD_OBJ_HWPT_NESTED.
- iommufd_fault for IOMMUFD_OBJ_FAULT.
- iommufd_viommu for IOMMUFD_OBJ_VIOMMU.
- iommufd_vdevice for IOMMUFD_OBJ_VDEVICE.
- iommufd_veventq for IOMMUFD_OBJ_VEVENTQ.
- iommufd_hw_queue for IOMMUFD_OBJ_HW_QUEUE.

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