[PATCH RFC 02/12] iommufd: Overview documentation

[Jason Gunthorpe <jgg@nvidia.com>]

From: Kevin Tian <kevin.tian@intel.com>

Add iommufd to the documentation tree.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 Documentation/userspace-api/index.rst   |   1 +
 Documentation/userspace-api/iommufd.rst | 224 ++++++++++++++++++++++++
 2 files changed, 225 insertions(+)
 create mode 100644 Documentation/userspace-api/iommufd.rst

diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
index a61eac0c73f825..3815f013e4aebd 100644
--- a/Documentation/userspace-api/index.rst
+++ b/Documentation/userspace-api/index.rst
@@ -25,6 +25,7 @@ place where this information is gathered.
    ebpf/index
    ioctl/index
    iommu
+   iommufd
    media/index
    sysfs-platform_profile
    vduse
diff --git a/Documentation/userspace-api/iommufd.rst b/Documentation/userspace-api/iommufd.rst
new file mode 100644
index 00000000000000..38035b3822fd23
--- /dev/null
+++ b/Documentation/userspace-api/iommufd.rst
@@ -0,0 +1,224 @@
+.. 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 that point at user space memory. It intends to be general and
+consumable by any driver that wants to DMA to userspace. Those drivers are
+expected to deprecate any proprietary IOMMU logic, if existing (e.g.
+vfio_iommu_type1.c).
+
+At minimum iommufd provides a 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 to
+run the user API over.
+
+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 copies its 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, wrapping 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 the
+  IOAS will synchronize its mapping with each member HW_PAGETABLE.
+
+All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
+
+Linkage between user-visible objects and external kernel datastructures are
+reflected by dotted line arrows below, with numbers referring to certain
+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. One 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 that are inside
+   it.
+
+2. IOMMUFD_OBJ_DEVICE is created when an external driver calls the IOMMUFD kAPI
+   to bind a device to an iommufd. The external driver is expected to implement
+   proper uAPI for userspace to initiate the binding operation. Successful
+   completion of this operation establishes the desired DMA ownership over the
+   device. The external driver must set 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 a new iommu_domain) is created. Successful
+   completion of this operation sets up the linkages among an IOAS, a device and
+   an 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 additions to IOMMUFD will provide an API to create and
+   manipulate the HW_PAGETABLE directly.
+
+One device can only bind to one iommufd (due to DMA ownership claim) and attach
+to at most one IOAS object (no support of PASID yet).
+
+Currently only PCI device is allowed.
+
+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, referring 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, referring to an iommu domain designated by the user as the
+  target pagetable to be attached to by a device. Though currently no user API
+  for userspace to directly create such domain, the datastructure and algorithms
+  are ready for that usage.
+
+- In-kernel user, referring to something like a VFIO mdev that is accessing the
+  IOAS and using a 'struct page \*' for CPU based access. Such users require an
+  isolation granularity smaller than what an iommu domain can afford. They must
+  manually enforce the IOAS constraints on DMA buffers before those buffers can
+  be accessed by mdev. Though no kernel API for an external driver to bind a
+  mdev, the datastructure and algorithms are ready for such usage.
+
+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_areas 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_user representing in-kernel users of PFNs
+- struct xarray pinned_pfns holding a list of pages pinned by
+   in-kernel Users
+
+The iopt_pages is the center of the storage and motion of PFNs. Each iopt_pages
+represents a logical linear array of full PFNs. PFNs are stored in a tiered
+scheme:
+
+ 1) iopt_pages::pinned_pfns xarray
+ 2) An iommu_domain
+ 3) The origin of the PFNs, i.e. the userspace pointer
+
+PFN have to be copied between all combinations of tiers, depending on the
+configuration (i.e. attached domains and in-kernel users).
+
+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 driver 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:
+
+VFIO and IOMMUFD
+----------------
+
+Connecting VFIO device to iommufd can be done in two approaches.
+
+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
-- 
2.35.1