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>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>

Documentation/driver-api/vfio.rst

@@ -239,6 +239,130 @@ group and can access them as follows::
 	/* Gratuitous device reset and go... */
 	ioctl(device, VFIO_DEVICE_RESET);
 
+IOMMUFD and vfio_iommu_type1
+----------------------------
+
+IOMMUFD is the new user API to manage I/O page tables from userspace.
+It intends to be the portal of delivering advanced userspace DMA
+features (nested translation [5], PASID [6], etc.) while also providing
+a backwards compatibility interface for existing VFIO_TYPE1v2_IOMMU use
+cases.  Eventually the vfio_iommu_type1 driver, as well as the legacy
+vfio container and group model is intended to be deprecated.
+
+The IOMMUFD backwards compatibility interface can be enabled two ways.
+In the first method, the kernel can be configured with
+CONFIG_IOMMUFD_VFIO_CONTAINER, in which case the IOMMUFD subsystem
+transparently provides the entire infrastructure for the VFIO
+container and IOMMU backend interfaces.  The compatibility mode can
+also be accessed if the VFIO container interface, ie. /dev/vfio/vfio is
+simply symlink'd to /dev/iommu.  Note that at the time of writing, the
+compatibility mode is not entirely feature complete relative to
+VFIO_TYPE1v2_IOMMU (ex. DMA mapping MMIO) and does not attempt to
+provide compatibility to the VFIO_SPAPR_TCE_IOMMU interface.  Therefore
+it is not generally advisable at this time to switch from native VFIO
+implementations to the IOMMUFD compatibility interfaces.
+
+Long term, VFIO users should migrate to device access through the cdev
+interface described below, and native access through the IOMMUFD
+provided interfaces.
+
+VFIO Device cdev
+----------------
+
+Traditionally user acquires a device fd via VFIO_GROUP_GET_DEVICE_FD
+in a VFIO group.
+
+With CONFIG_VFIO_DEVICE_CDEV=y the user can now acquire a device fd
+by directly opening a character device /dev/vfio/devices/vfioX where
+"X" is the number allocated uniquely by VFIO for registered devices.
+For noiommu devices, the character device would be named with "noiommu-"
+prefix. e.g. /dev/vfio/devices/noiommu-vfioX.
+
+The cdev only works with IOMMUFD.  Both VFIO drivers and applications
+must adapt to the new cdev security model which requires using
+VFIO_DEVICE_BIND_IOMMUFD to claim DMA ownership before starting to
+actually use the device.  Once BIND succeeds then a VFIO device can
+be fully accessed by the user.
+
+VFIO device cdev doesn't rely on VFIO group/container/iommu drivers.
+Hence those modules can be fully compiled out in an environment
+where no legacy VFIO application exists.
+
+So far SPAPR does not support IOMMUFD yet.  So it cannot support device
+cdev neither.
+
+Device cdev Example
+-------------------
+
+Assume user wants to access PCI device 0000:6a:01.0::
+
+	$ ls /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/
+	vfio0
+
+This device is therefore represented as vfio0.  The user can verify
+its existence::
+
+	$ ls -l /dev/vfio/devices/vfio0
+	crw------- 1 root root 511, 0 Feb 16 01:22 /dev/vfio/devices/vfio0
+	$ cat /sys/bus/pci/devices/0000:6a:01.0/vfio-dev/vfio0/dev
+	511:0
+	$ ls -l /dev/char/511\:0
+	lrwxrwxrwx 1 root root 21 Feb 16 01:22 /dev/char/511:0 -> ../vfio/devices/vfio0
+
+Then provide the user with access to the device if unprivileged
+operation is desired::
+
+	$ chown user:user /dev/vfio/devices/vfio0
+
+Finally the user could get cdev fd by::
+
+	cdev_fd = open("/dev/vfio/devices/vfio0", O_RDWR);
+
+An opened cdev_fd doesn't give the user any permission of accessing
+the device except binding the cdev_fd to an iommufd.  After that point
+then the device is fully accessible including attaching it to an
+IOMMUFD IOAS/HWPT to enable userspace DMA::
+
+	struct vfio_device_bind_iommufd bind = {
+		.argsz = sizeof(bind),
+		.flags = 0,
+	};
+	struct iommu_ioas_alloc alloc_data  = {
+		.size = sizeof(alloc_data),
+		.flags = 0,
+	};
+	struct vfio_device_attach_iommufd_pt attach_data = {
+		.argsz = sizeof(attach_data),
+		.flags = 0,
+	};
+	struct iommu_ioas_map map = {
+		.size = sizeof(map),
+		.flags = IOMMU_IOAS_MAP_READABLE |
+			 IOMMU_IOAS_MAP_WRITEABLE |
+			 IOMMU_IOAS_MAP_FIXED_IOVA,
+		.__reserved = 0,
+	};
+
+	iommufd = open("/dev/iommu", O_RDWR);
+
+	bind.iommufd = iommufd; // negative value means vfio-noiommu mode
+	ioctl(cdev_fd, VFIO_DEVICE_BIND_IOMMUFD, &bind);
+
+	ioctl(iommufd, IOMMU_IOAS_ALLOC, &alloc_data);
+	attach_data.pt_id = alloc_data.out_ioas_id;
+	ioctl(cdev_fd, VFIO_DEVICE_ATTACH_IOMMUFD_PT, &attach_data);
+
+	/* Allocate some space and setup a DMA mapping */
+	map.user_va = (int64_t)mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
+				    MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
+	map.iova = 0; /* 1MB starting at 0x0 from device view */
+	map.length = 1024 * 1024;
+	map.ioas_id = alloc_data.out_ioas_id;;
+
+	ioctl(iommufd, IOMMU_IOAS_MAP, &map);
+
+	/* Other device operations as stated in "VFIO Usage Example" */
+
 VFIO User API
 -------------------------------------------------------------------------------
 
@@ -566,3 +690,11 @@ This implementation has some specifics:
 				\-0d.1
 
 	00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
+
+.. [5] Nested translation is an IOMMU feature which supports two stage
+   address translations.  This improves the address translation efficiency
+   in IOMMU virtualization.
+
+.. [6] PASID stands for Process Address Space ID, introduced by PCI
+   Express.  It is a prerequisite for Shared Virtual Addressing (SVA)
+   and Scalable I/O Virtualization (Scalable IOV).