iommufd: Document overview of iommufd

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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