1.. SPDX-License-Identifier: GPL-2.0+
2
3=======
4IOMMUFD
5=======
6
7:Author: Jason Gunthorpe
8:Author: Kevin Tian
9
10Overview
11========
12
13IOMMUFD is the user API to control the IOMMU subsystem as it relates to managing
14IO page tables from userspace using file descriptors. It intends to be general
15and consumable by any driver that wants to expose DMA to userspace. These
16drivers are eventually expected to deprecate any internal IOMMU logic
17they may already/historically implement (e.g. vfio_iommu_type1.c).
18
19At minimum iommufd provides universal support of managing I/O address spaces and
20I/O page tables for all IOMMUs, with room in the design to add non-generic
21features to cater to specific hardware functionality.
22
23In this context the capital letter (IOMMUFD) refers to the subsystem while the
24small letter (iommufd) refers to the file descriptors created via /dev/iommu for
25use by userspace.
26
27Key Concepts
28============
29
30User Visible Objects
31--------------------
32
33Following IOMMUFD objects are exposed to userspace:
34
35- IOMMUFD_OBJ_IOAS, representing an I/O address space (IOAS), allowing map/unmap
36  of user space memory into ranges of I/O Virtual Address (IOVA).
37
38  The IOAS is a functional replacement for the VFIO container, and like the VFIO
39  container it copies an IOVA map to a list of iommu_domains held within it.
40
41- IOMMUFD_OBJ_DEVICE, representing a device that is bound to iommufd by an
42  external driver.
43
44- IOMMUFD_OBJ_HW_PAGETABLE, representing an actual hardware I/O page table
45  (i.e. a single struct iommu_domain) managed by the iommu driver.
46
47  The IOAS has a list of HW_PAGETABLES that share the same IOVA mapping and
48  it will synchronize its mapping with each member HW_PAGETABLE.
49
50All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
51
52The diagram below shows relationship between user-visible objects and kernel
53datastructures (external to iommufd), with numbers referred to operations
54creating the objects and links::
55
56  _________________________________________________________
57 |                         iommufd                         |
58 |       [1]                                               |
59 |  _________________                                      |
60 | |                 |                                     |
61 | |                 |                                     |
62 | |                 |                                     |
63 | |                 |                                     |
64 | |                 |                                     |
65 | |                 |                                     |
66 | |                 |        [3]                 [2]      |
67 | |                 |    ____________         __________  |
68 | |      IOAS       |<--|            |<------|          | |
69 | |                 |   |HW_PAGETABLE|       |  DEVICE  | |
70 | |                 |   |____________|       |__________| |
71 | |                 |         |                   |       |
72 | |                 |         |                   |       |
73 | |                 |         |                   |       |
74 | |                 |         |                   |       |
75 | |                 |         |                   |       |
76 | |_________________|         |                   |       |
77 |         |                   |                   |       |
78 |_________|___________________|___________________|_______|
79           |                   |                   |
80           |              _____v______      _______v_____
81           | PFN storage |            |    |             |
82           |------------>|iommu_domain|    |struct device|
83                         |____________|    |_____________|
84
851. IOMMUFD_OBJ_IOAS is created via the IOMMU_IOAS_ALLOC uAPI. An iommufd can
86   hold multiple IOAS objects. IOAS is the most generic object and does not
87   expose interfaces that are specific to single IOMMU drivers. All operations
88   on the IOAS must operate equally on each of the iommu_domains inside of it.
89
902. IOMMUFD_OBJ_DEVICE is created when an external driver calls the IOMMUFD kAPI
91   to bind a device to an iommufd. The driver is expected to implement a set of
92   ioctls to allow userspace to initiate the binding operation. Successful
93   completion of this operation establishes the desired DMA ownership over the
94   device. The driver must also set the driver_managed_dma flag and must not
95   touch the device until this operation succeeds.
96
973. IOMMUFD_OBJ_HW_PAGETABLE is created when an external driver calls the IOMMUFD
98   kAPI to attach a bound device to an IOAS. Similarly the external driver uAPI
99   allows userspace to initiate the attaching operation. If a compatible
100   pagetable already exists then it is reused for the attachment. Otherwise a
101   new pagetable object and iommu_domain is created. Successful completion of
102   this operation sets up the linkages among IOAS, device and iommu_domain. Once
103   this completes the device could do DMA.
104
105   Every iommu_domain inside the IOAS is also represented to userspace as a
106   HW_PAGETABLE object.
107
108   .. note::
109
110      Future IOMMUFD updates will provide an API to create and manipulate the
111      HW_PAGETABLE directly.
112
113A device can only bind to an iommufd due to DMA ownership claim and attach to at
114most one IOAS object (no support of PASID yet).
115
116Kernel Datastructure
117--------------------
118
119User visible objects are backed by following datastructures:
120
121- iommufd_ioas for IOMMUFD_OBJ_IOAS.
122- iommufd_device for IOMMUFD_OBJ_DEVICE.
123- iommufd_hw_pagetable for IOMMUFD_OBJ_HW_PAGETABLE.
124
125Several terminologies when looking at these datastructures:
126
127- Automatic domain - refers to an iommu domain created automatically when
128  attaching a device to an IOAS object. This is compatible to the semantics of
129  VFIO type1.
130
131- Manual domain - refers to an iommu domain designated by the user as the
132  target pagetable to be attached to by a device. Though currently there are
133  no uAPIs to directly create such domain, the datastructure and algorithms
134  are ready for handling that use case.
135
136- In-kernel user - refers to something like a VFIO mdev that is using the
137  IOMMUFD access interface to access the IOAS. This starts by creating an
138  iommufd_access object that is similar to the domain binding a physical device
139  would do. The access object will then allow converting IOVA ranges into struct
140  page * lists, or doing direct read/write to an IOVA.
141
142iommufd_ioas serves as the metadata datastructure to manage how IOVA ranges are
143mapped to memory pages, composed of:
144
145- struct io_pagetable holding the IOVA map
146- struct iopt_area's representing populated portions of IOVA
147- struct iopt_pages representing the storage of PFNs
148- struct iommu_domain representing the IO page table in the IOMMU
149- struct iopt_pages_access representing in-kernel users of PFNs
150- struct xarray pinned_pfns holding a list of pages pinned by in-kernel users
151
152Each iopt_pages represents a logical linear array of full PFNs. The PFNs are
153ultimately derived from userspace VAs via an mm_struct. Once they have been
154pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pfns
155xarray if they have been pinned through an iommufd_access.
156
157PFN have to be copied between all combinations of storage locations, depending
158on what domains are present and what kinds of in-kernel "software access" users
159exist. The mechanism ensures that a page is pinned only once.
160
161An io_pagetable is composed of iopt_areas pointing at iopt_pages, along with a
162list of iommu_domains that mirror the IOVA to PFN map.
163
164Multiple io_pagetable-s, through their iopt_area-s, can share a single
165iopt_pages which avoids multi-pinning and double accounting of page
166consumption.
167
168iommufd_ioas is sharable between subsystems, e.g. VFIO and VDPA, as long as
169devices managed by different subsystems are bound to a same iommufd.
170
171IOMMUFD User API
172================
173
174.. kernel-doc:: include/uapi/linux/iommufd.h
175
176IOMMUFD Kernel API
177==================
178
179The IOMMUFD kAPI is device-centric with group-related tricks managed behind the
180scene. This allows the external drivers calling such kAPI to implement a simple
181device-centric uAPI for connecting its device to an iommufd, instead of
182explicitly imposing the group semantics in its uAPI as VFIO does.
183
184.. kernel-doc:: drivers/iommu/iommufd/device.c
185   :export:
186
187.. kernel-doc:: drivers/iommu/iommufd/main.c
188   :export:
189
190VFIO and IOMMUFD
191----------------
192
193Connecting a VFIO device to iommufd can be done in two ways.
194
195First is a VFIO compatible way by directly implementing the /dev/vfio/vfio
196container IOCTLs by mapping them into io_pagetable operations. Doing so allows
197the use of iommufd in legacy VFIO applications by symlinking /dev/vfio/vfio to
198/dev/iommufd or extending VFIO to SET_CONTAINER using an iommufd instead of a
199container fd.
200
201The second approach directly extends VFIO to support a new set of device-centric
202user API based on aforementioned IOMMUFD kernel API. It requires userspace
203change but better matches the IOMMUFD API semantics and easier to support new
204iommufd features when comparing it to the first approach.
205
206Currently both approaches are still work-in-progress.
207
208There are still a few gaps to be resolved to catch up with VFIO type1, as
209documented in iommufd_vfio_check_extension().
210
211Future TODOs
212============
213
214Currently IOMMUFD supports only kernel-managed I/O page table, similar to VFIO
215type1. New features on the radar include:
216
217 - Binding iommu_domain's to PASID/SSID
218 - Userspace page tables, for ARM, x86 and S390
219 - Kernel bypass'd invalidation of user page tables
220 - Re-use of the KVM page table in the IOMMU
221 - Dirty page tracking in the IOMMU
222 - Runtime Increase/Decrease of IOPTE size
223 - PRI support with faults resolved in userspace
224