[PATCH v2 08/13] vfio: Add documentation

Alex Williamson
2012-05-22T05:05:29Z

Documentation/vfio.txt | 315 ++++++++++++++++++++++++++++++++++++++++++++++++
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create mode 100644 Documentation/vfio.txt

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+VFIO - "Virtual Function I/O"[1]
+-------------------------------------------------------------------------------
+Many modern system now provide DMA and interrupt remapping facilities
+to help ensure I/O devices behave within the boundaries they've been
+allotted. This includes x86 hardware with AMD-Vi and Intel VT-d,
+POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
+systems such as Freescale PAMU. The VFIO driver is an IOMMU/device
+agnostic framework for exposing direct device access to userspace, in
+a secure, IOMMU protected environment. In other words, this allows
+safe[2], non-privileged, userspace drivers.
+
+Why do we want that? Virtual machines often make use of direct device
+access ("device assignment") when configured for the highest possible
+I/O performance. From a device and host perspective, this simply
+turns the VM into a userspace driver, with the benefits of
+significantly reduced latency, higher bandwidth, and direct use of
+bare-metal device drivers[3].
+
+Some applications, particularly in the high performance computing
+field, also benefit from low-overhead, direct device access from
+userspace. Examples include network adapters (often non-TCP/IP based)
+and compute accelerators. Prior to VFIO, these drivers had to either
+go through the full development cycle to become proper upstream
+driver, be maintained out of tree, or make use of the UIO framework,
+which has no notion of IOMMU protection, limited interrupt support,
+and requires root privileges to access things like PCI configuration
+space.
+
+The VFIO driver framework intends to unify these, replacing both the
+KVM PCI specific device assignment code as well as provide a more
+secure, more featureful userspace driver environment than UIO.
+
+Groups, Devices, and IOMMUs
+-------------------------------------------------------------------------------
+
+Devices are the main target of any I/O driver. Devices typically
+create a programming interface made up of I/O access, interrupts,
+and DMA. Without going into the details of each of these, DMA is
+by far the most critical aspect for maintaining a secure environment
+as allowing a device read-write access to system memory imposes the
+greatest risk to the overall system integrity.
+
+To help mitigate this risk, many modern IOMMUs now incorporate
+isolation properties into what was, in many cases, an interface only
+meant for translation (ie. solving the addressing problems of devices
+with limited address spaces). With this, devices can now be isolated
+from each other and from arbitrary memory access, thus allowing
+things like secure direct assignment of devices into virtual machines.
+
+This isolation is not always at the granularity of a single device
+though. Even when an IOMMU is capable of this, properties of devices,
+interconnects, and IOMMU topologies can each reduce this isolation.
+For instance, an individual device may be part of a larger multi-
+function enclosure. While the IOMMU may be able to distinguish
+between devices within the enclosure, the enclosure may not require
+transactions between devices to reach the IOMMU. Examples of this
+could be anything from a multi-function PCI device with backdoors
+between functions to a non-PCI-ACS (Access Control Services) capable
+bridge allowing redirection without reaching the IOMMU. Topology
+can also play a factor in terms of hiding devices. A PCIe-to-PCI
+bridge masks the devices behind it, making transaction appear as if
+from the bridge itself. Obviously IOMMU design plays a major factor
+as well.
+
+Therefore, while for the most part an IOMMU may have device level
+granularity, any system is susceptible to reduced granularity. The
+IOMMU API therefore supports a notion of IOMMU groups. A group is
+a set of devices which is isolatable from all other devices in the
+system. Groups are therefore the unit of ownership used by VFIO.
+
+While the group is the minimum granularity that must be used to
+ensure secure user access, it's not necessarily the preferred
+granularity. In IOMMUs which make use of page tables, it may be
+possible to share a set of page tables between different groups,
+reducing the overhead both to the platform (reduced TLB thrashing,
+reduced duplicate page tables), and to the user (programming only
+a single set of translations). For this reason, VFIO makes use of
+a container class, which may hold one or more groups. A container
+is created by simply opening the /dev/vfio/vfio character device.
+
+On its own, the container provides little functionality, with all
+but a couple version and extension query interfaces locked away.
+The user needs to add a group into the container for the next level
+of functionality. To do this, the user first needs to identify the
+group associated with the desired device. This can be done using
+the sysfs links described in the example below. By unbinding the
+device from the host driver and binding it to a VFIO driver, a new
+VFIO group will appear for the group as /dev/vfio/$GROUP, where
+$GROUP is the IOMMU group number of which the device is a member.
+If the IOMMU group contains multiple devices, each will need to
+be bound to a VFIO driver before operations on the VFIO group
+are allowed (it's also sufficient to only unbind the device from
+host drivers if a VFIO driver is unavailable; this will make the
+group available, but not that particular device). TBD - interface
+for disabling driver probing/locking a device.
+
+Once the group is ready, it may be added to the container by opening
+the VFIO group character device (/dev/vfio/$GROUP) and using the
+VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
+previously opened container file. If desired and if the IOMMU driver
+supports sharing the IOMMU context between groups, multiple groups may
+be set to the same container. If a group fails to set to a container
+with existing groups, a new empty container will need to be used
+instead.
+
+With a group (or groups) attached to a container, the remaining
+ioctls become available, enabling access to the VFIO IOMMU interfaces.
+Additionally, it now becomes possible to get file descriptors for each
+device within a group using an ioctl on the VFIO group file descriptor.
+
+The VFIO device API includes ioctls for describing the device, the I/O
+regions and their read/write/mmap offsets on the device descriptor, as
+well as mechanisms for describing and registering interrupt
+notifications.
+
+VFIO Usage Example
+-------------------------------------------------------------------------------
+
+Assume user wants to access PCI device 0000:06:0d.0
+
+$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
+../../../../kernel/iommu_groups/26
+
+This device is therefore in IOMMU group 26. This device is on the
+pci bus, therefore the user will make use of vfio-pci to manage the
+group:
+
+# modprobe vfio-pci
+
+Binding this device to the vfio-pci driver creates the VFIO group
+character devices for this group:
+
+$ lspci -n -s 0000:06:0d.0
+06:0d.0 0401: 1102:0002 (rev 08)
+# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
+# echo 1102 0002 > /sys/bus/pci/drivers/vfio/new_id
+# echo 0000:06:0d.0 > /sys/bus/pci/drivers/vfio/bind
+
+Now we need to look at what other devices are in the group to free
+it for use by VFIO:
+
+$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
+total 0
+lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
+ ../../../../devices/pci0000:00/0000:00:1e.0
+lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
+ ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
+lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.1 ->
+ ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.1
+
+This device is behind a PCIe-to-PCI bridge[4], therefore we also
+need to add device 0000:06:0d.1 to the group following the same
+procedure as above. Device 0000:00:1e.0 is a bridge that does
+not currently have a host driver, therefore it's not required to
+bind this device to the vfio-pci driver (vfio-pci does not currently
+support PCI bridges).
+
+The final step is to provide the user with access to the group if
+unprivileged operation is desired (note that /dev/vfio/vfio provides
+no capabilities on its own and is therefore expected to be set to
+mode 0666 by the system).
+
+# chown user:user /dev/vfio/26
+
+The user now has full access to all the devices and the iommu for this
+group and can access them as follows:
+
+ int container, group, device, i;
+ struct vfio_group_status group_status =
+ { .argsz = sizeof(group_status) };
+ struct vfio_iommu_x86_info iommu_info = { .argsz = sizeof(iommu_info) };
+ struct vfio_iommu_x86_dma_map dma_map = { .argsz = sizeof(dma_map) };
+ struct vfio_device_info device_info = { .argsz = sizeof(device_info) };
+
+ /* Create a new container */
+ container = open("/dev/vfio/vfio, O_RDWR);
+
+ if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
+ /* Unknown API version */
+
+ if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_X86_IOMMU))
+ /* Doesn't support the IOMMU driver we want. */
+
+ /* Open the group */
+ group = open("/dev/vfio/26", O_RDWR);
+
+ /* Test the group is viable and available */
+ ioctl(group, VFIO_GROUP_GET_STATUS, &group_status);
+
+ if (!(group_status.flags & VFIO_GROUP_FLAGS_VIABLE))
+ /* Group is not viable (ie, not all devices bound for vfio) */
+
+ /* Add the group to the container */
+ ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
+
+ /* Enable the IOMMU model we want */
+ ioctl(container, VFIO_SET_IOMMU, VFIO_X86_IOMMU)
+
+ /* Get addition IOMMU info */
+ ioctl(container, VFIO_IOMMU_GET_INFO, &iommu_info);
+
+ /* Allocate some space and setup a DMA mapping */
+ dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
+ dma_map.size = 1024 * 1024;
+ dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
+ dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
+
+ ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
+
+ /* Get a file descriptor for the device */
+ device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
+
+ /* Test and setup the device */
+ ioctl(device, VFIO_DEVICE_GET_INFO, &device_info);
+
+ for (i = 0; i < device_info.num_regions; i++) {
+ struct vfio_region_info reg = { .argsz = sizeof(reg) };
+
+ reg.index = i;
+
+ ioctl(device, VFIO_DEVICE_GET_REGION_INFO, &reg);
+
+ /* Setup mappings... read/write offsets, mmaps
+ * For PCI devices, config space is a region */
+ }
+
+ for (i = 0; i < device_info.num_irqs; i++) {
+ struct vfio_irq_info irq = { .argsz = sizeof(irq) };
+
+ irq.index = i;
+
+ ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &reg);
+
+ /* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
+ }
+
+ /* Gratuitous device reset and go... */
+ ioctl(device, VFIO_DEVICE_RESET);
+
+VFIO User API
+-------------------------------------------------------------------------------
+
+Please see include/linux/vfio.h for complete API documentation.
+
+VFIO bus driver API
+-------------------------------------------------------------------------------
+
+VFIO bus drivers, such as vfio-pci make use of only a few interfaces
+into VFIO core. When devices are bound and unbound to the driver,
+the driver should call vfio_add_group_dev() and vfio_del_group_dev()
+respectively:
+
+extern int vfio_add_group_dev(struct iommu_group *iommu_group,
+ struct device *dev,
+ const struct vfio_device_ops *ops,
+ void *device_data);
+
+extern void *vfio_del_group_dev(struct device *dev);
+
+vfio_add_group_dev() indicates to the core to begin tracking the
+specified iommu_group and register the specified dev as owned by
+a VFIO bus driver. The driver provides an ops structure for callbacks
+similar to a file operations structure:
+
+struct vfio_device_ops {
+ int (*open)(void *device_data);
+ void (*release)(void *device_data);
+ ssize_t (*read)(void *device_data, char __user *buf,
+ size_t count, loff_t *ppos);
+ ssize_t (*write)(void *device_data, const char __user *buf,
+ size_t size, loff_t *ppos);
+ long (*ioctl)(void *device_data, unsigned int cmd,
+ unsigned long arg);
+ int (*mmap)(void *device_data, struct vm_area_struct *vma);
+};
+
+Each function is passed the device_data that was originally registered
+in the vfio_add_group_dev() call above. This allows the bus driver
+an easy place to store its opaque, private data. The open/release
+callbacks are issued when a new file descriptor is created for a
+device (via VFIO_GROUP_GET_DEVICE_FD). The ioctl interface provides
+a direct pass through for VFIO_DEVICE_* ioctls. The read/write/mmap
+interfaces implement the device region access defined by the device's
+own VFIO_DEVICE_GET_REGION_INFO ioctl.
+
+-------------------------------------------------------------------------------
+
+[1] VFIO was originally an acronym for "Virtual Function I/O" in its
+initial implementation by Tom Lyon while as Cisco. We've since
+outgrown the acronym, but it's catchy.
+
+[2] "safe" also depends upon a device being "well behaved". It's
+possible for multi-function devices to have backdoors between
+functions and even for single function devices to have alternative
+access to things like PCI config space through MMIO registers. To
+guard against the former we can include additional precautions in the
+IOMMU driver to group multi-function PCI devices together
+(iommu=group_mf). The latter we can't prevent, but the IOMMU should
+still provide isolation. For PCI, SR-IOV Virtual Functions are the
+best indicator of "well behaved", as these are designed for
+virtualization usage models.
+
+[3] As always there are trade-offs to virtual machine device
+assignment that are beyond the scope of VFIO. It's expected that
+future IOMMU technologies will reduce some, but maybe not all, of
+these trade-offs.
+
+[4] In this case the device is below a PCI bridge, so transactions
+from either function of the device are indistinguishable to the iommu:
+
+-[0000:00]-+-1e.0-[06]--+-0d.0
+ \-0d.1
+
+00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)