Hi, On 11/16/22 22:00, Jason Gunthorpe wrote: > From: Kevin Tian <kevin.tian@xxxxxxxxx> > > Add iommufd into the documentation tree, and supply initial documentation. > Much of this is linked from code comments by kdoc. > > Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> > Signed-off-by: Kevin Tian <kevin.tian@xxxxxxxxx> > Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > --- > Documentation/userspace-api/index.rst | 1 + > Documentation/userspace-api/iommufd.rst | 223 ++++++++++++++++++++++++ > 2 files changed, 224 insertions(+) > create mode 100644 Documentation/userspace-api/iommufd.rst > > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst > index c78da9ce0ec44e..f16337bdb8520f 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 > netlink/index > sysfs-platform_profile > diff --git a/Documentation/userspace-api/iommufd.rst b/Documentation/userspace-api/iommufd.rst > new file mode 100644 > index 00000000000000..8b1392fd2e3487 > --- /dev/null > +++ b/Documentation/userspace-api/iommufd.rst > @@ -0,0 +1,223 @@ > +.. 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_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_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 userspave VAs via an mm_struct. Once they have been > +pinned the PFNs are stored in IOPTEs of an iommu_domain or inside the pinned_pages > +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 > +exists. 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 Reviewed-by: Eric Auger <eric.auger@xxxxxxxxxx> Eric
