1 .. SPDX-License-Identifier: GPL-2.0+ 2 3 ======= 4 IOMMUFD 5 ======= 6 7 :Author: Jason Gunthorpe 8 :Author: Kevin Tian 9 10 Overview 11 ======== 12 13 IOMMUFD is the user API to control the IOMMU subsystem as it relates to managing 14 IO page tables from userspace using file descriptors. It intends to be general 15 and consumable by any driver that wants to expose DMA to userspace. These 16 drivers are eventually expected to deprecate any internal IOMMU logic 17 they may already/historically implement (e.g. vfio_iommu_type1.c). 18 19 At minimum iommufd provides universal support of managing I/O address spaces and 20 I/O page tables for all IOMMUs, with room in the design to add non-generic 21 features to cater to specific hardware functionality. 22 23 In this context the capital letter (IOMMUFD) refers to the subsystem while the 24 small letter (iommufd) refers to the file descriptors created via /dev/iommu for 25 use by userspace. 26 27 Key Concepts 28 ============ 29 30 User Visible Objects 31 -------------------- 32 33 Following 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 50 All user-visible objects are destroyed via the IOMMU_DESTROY uAPI. 51 52 The diagram below shows relationship between user-visible objects and kernel 53 datastructures (external to iommufd), with numbers referred to operations 54 creating 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 85 1. 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 90 2. 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 97 3. 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. 112 113 A device can only bind to an iommufd due to DMA ownership claim and attach to at 114 most one IOAS object (no support of PASID yet). 115 116 Kernel Datastructure 117 -------------------- 118 119 User 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. 124 125 Several 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. 141 142 iommufd_ioas serves as the metadata datastructure to manage how IOVA ranges are 143 mapped 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 151 152 Each iopt_pages represents a logical linear array of full PFNs. The PFNs are 153 ultimately derived from userspace VAs via an mm_struct. Once they have been 154 pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pfns 155 xarray if they have been pinned through an iommufd_access. 156 157 PFN have to be copied between all combinations of storage locations, depending 158 on what domains are present and what kinds of in-kernel "software access" users 159 exist. The mechanism ensures that a page is pinned only once. 160 161 An io_pagetable is composed of iopt_areas pointing at iopt_pages, along with a 162 list of iommu_domains that mirror the IOVA to PFN map. 163 164 Multiple io_pagetable-s, through their iopt_area-s, can share a single 165 iopt_pages which avoids multi-pinning and double accounting of page 166 consumption. 167 168 iommufd_ioas is shareable between subsystems, e.g. VFIO and VDPA, as long as 169 devices managed by different subsystems are bound to a same iommufd. 170 171 IOMMUFD User API 172 ================ 173 174 .. kernel-doc:: include/uapi/linux/iommufd.h 175 176 IOMMUFD Kernel API 177 ================== 178 179 The IOMMUFD kAPI is device-centric with group-related tricks managed behind the 180 scene. This allows the external drivers calling such kAPI to implement a simple 181 device-centric uAPI for connecting its device to an iommufd, instead of 182 explicitly 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: 189 190 VFIO and IOMMUFD 191 ---------------- 192 193 Connecting a VFIO device to iommufd can be done in two ways. 194 195 First is a VFIO compatible way by directly implementing the /dev/vfio/vfio 196 container IOCTLs by mapping them into io_pagetable operations. Doing so allows 197 the 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 199 container fd. 200 201 The second approach directly extends VFIO to support a new set of device-centric 202 user API based on aforementioned IOMMUFD kernel API. It requires userspace 203 change but better matches the IOMMUFD API semantics and easier to support new 204 iommufd features when comparing it to the first approach. 205 206 Currently both approaches are still work-in-progress. 207 208 There are still a few gaps to be resolved to catch up with VFIO type1, as 209 documented in iommufd_vfio_check_extension(). 210 211 Future TODOs 212 ============ 213 214 Currently IOMMUFD supports only kernel-managed I/O page table, similar to VFIO 215 type1. 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
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