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docs: vfio: Add vfio device cdev description
This gives notes for userspace applications on device cdev usage. Reviewed-by: Kevin Tian <kevin.tian@intel.com> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Tested-by: Yanting Jiang <yanting.jiang@intel.com> Signed-off-by: Yi Liu <yi.l.liu@intel.com> Link: https://lore.kernel.org/r/20230718135551.6592-27-yi.l.liu@intel.com Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
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Yi Liu
Alex Williamson
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@ -239,6 +239,137 @@ group and can access them as follows::
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/* Gratuitous device reset and go... */
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ioctl(device, VFIO_DEVICE_RESET);
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IOMMUFD and vfio_iommu_type1
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----------------------------
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IOMMUFD is the new user API to manage I/O page tables from userspace.
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It intends to be the portal of delivering advanced userspace DMA
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features (nested translation [5]_, PASID [6]_, etc.) while also providing
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a backwards compatibility interface for existing VFIO_TYPE1v2_IOMMU use
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cases. Eventually the vfio_iommu_type1 driver, as well as the legacy
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vfio container and group model is intended to be deprecated.
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The IOMMUFD backwards compatibility interface can be enabled two ways.
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In the first method, the kernel can be configured with
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CONFIG_IOMMUFD_VFIO_CONTAINER, in which case the IOMMUFD subsystem
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transparently provides the entire infrastructure for the VFIO
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container and IOMMU backend interfaces. The compatibility mode can
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also be accessed if the VFIO container interface, ie. /dev/vfio/vfio is
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simply symlink'd to /dev/iommu. Note that at the time of writing, the
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compatibility mode is not entirely feature complete relative to
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VFIO_TYPE1v2_IOMMU (ex. DMA mapping MMIO) and does not attempt to
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provide compatibility to the VFIO_SPAPR_TCE_IOMMU interface. Therefore
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it is not generally advisable at this time to switch from native VFIO
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implementations to the IOMMUFD compatibility interfaces.
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Long term, VFIO users should migrate to device access through the cdev
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interface described below, and native access through the IOMMUFD
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provided interfaces.
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VFIO Device cdev
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----------------
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Traditionally user acquires a device fd via VFIO_GROUP_GET_DEVICE_FD
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in a VFIO group.
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With CONFIG_VFIO_DEVICE_CDEV=y the user can now acquire a device fd
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by directly opening a character device /dev/vfio/devices/vfioX where
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"X" is the number allocated uniquely by VFIO for registered devices.
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cdev interface does not support noiommu devices, so user should use
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the legacy group interface if noiommu is wanted.
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The cdev only works with IOMMUFD. Both VFIO drivers and applications
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must adapt to the new cdev security model which requires using
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VFIO_DEVICE_BIND_IOMMUFD to claim DMA ownership before starting to
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actually use the device. Once BIND succeeds then a VFIO device can
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be fully accessed by the user.
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VFIO device cdev doesn't rely on VFIO group/container/iommu drivers.
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Hence those modules can be fully compiled out in an environment
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where no legacy VFIO application exists.
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So far SPAPR does not support IOMMUFD yet. So it cannot support device
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cdev either.
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vfio device cdev access is still bound by IOMMU group semantics, ie. there
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can be only one DMA owner for the group. Devices belonging to the same
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group can not be bound to multiple iommufd_ctx or shared between native
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kernel and vfio bus driver or other driver supporting the driver_managed_dma
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flag. A violation of this ownership requirement will fail at the
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VFIO_DEVICE_BIND_IOMMUFD ioctl, which gates full device access.
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Device cdev Example
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-------------------
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Assume user wants to access PCI device 0000:6a:01.0::
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$ ls /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/
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vfio0
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This device is therefore represented as vfio0. The user can verify
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its existence::
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$ ls -l /dev/vfio/devices/vfio0
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crw------- 1 root root 511, 0 Feb 16 01:22 /dev/vfio/devices/vfio0
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$ cat /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/vfio0/dev
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511:0
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$ ls -l /dev/char/511\:0
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lrwxrwxrwx 1 root root 21 Feb 16 01:22 /dev/char/511:0 -> ../vfio/devices/vfio0
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Then provide the user with access to the device if unprivileged
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operation is desired::
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$ chown user:user /dev/vfio/devices/vfio0
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Finally the user could get cdev fd by::
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cdev_fd = open("/dev/vfio/devices/vfio0", O_RDWR);
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An opened cdev_fd doesn't give the user any permission of accessing
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the device except binding the cdev_fd to an iommufd. After that point
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then the device is fully accessible including attaching it to an
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IOMMUFD IOAS/HWPT to enable userspace DMA::
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struct vfio_device_bind_iommufd bind = {
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.argsz = sizeof(bind),
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.flags = 0,
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};
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struct iommu_ioas_alloc alloc_data = {
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.size = sizeof(alloc_data),
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.flags = 0,
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};
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struct vfio_device_attach_iommufd_pt attach_data = {
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.argsz = sizeof(attach_data),
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.flags = 0,
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};
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struct iommu_ioas_map map = {
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.size = sizeof(map),
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.flags = IOMMU_IOAS_MAP_READABLE |
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IOMMU_IOAS_MAP_WRITEABLE |
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IOMMU_IOAS_MAP_FIXED_IOVA,
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.__reserved = 0,
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};
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iommufd = open("/dev/iommu", O_RDWR);
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bind.iommufd = iommufd;
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ioctl(cdev_fd, VFIO_DEVICE_BIND_IOMMUFD, &bind);
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ioctl(iommufd, IOMMU_IOAS_ALLOC, &alloc_data);
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attach_data.pt_id = alloc_data.out_ioas_id;
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ioctl(cdev_fd, VFIO_DEVICE_ATTACH_IOMMUFD_PT, &attach_data);
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/* Allocate some space and setup a DMA mapping */
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map.user_va = (int64_t)mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
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map.iova = 0; /* 1MB starting at 0x0 from device view */
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map.length = 1024 * 1024;
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map.ioas_id = alloc_data.out_ioas_id;;
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ioctl(iommufd, IOMMU_IOAS_MAP, &map);
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/* Other device operations as stated in "VFIO Usage Example" */
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VFIO User API
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-------------------------------------------------------------------------------
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@ -566,3 +697,11 @@ This implementation has some specifics:
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\-0d.1
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00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
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.. [5] Nested translation is an IOMMU feature which supports two stage
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address translations. This improves the address translation efficiency
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in IOMMU virtualization.
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.. [6] PASID stands for Process Address Space ID, introduced by PCI
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Express. It is a prerequisite for Shared Virtual Addressing (SVA)
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and Scalable I/O Virtualization (Scalable IOV).
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