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    Subject[PATCH v2 08/13] vfio: Add documentation
    Signed-off-by: Alex Williamson <>

    Documentation/vfio.txt | 315 ++++++++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 315 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/vfio.txt

    diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
    new file mode 100644
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    +++ b/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
    +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/$GROUP) and using the
    +VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
    +previously opened container file. If desired and if the IOMMU driver
    +supports sharing the IOMMU context between groups, multiple groups may
    +be set to the same container. If a group fails to set to a container
    +with existing groups, a new empty container will need to be used
    +With a group (or groups) attached to a container, the remaining
    +ioctls become available, enabling access to the VFIO IOMMU interfaces.
    +Additionally, it now becomes possible to get file descriptors for each
    +device within a group using an ioctl on the VFIO group file descriptor.
    +The VFIO device API includes ioctls for describing the device, the I/O
    +regions and their read/write/mmap offsets on the device descriptor, as
    +well as mechanisms for describing and registering interrupt
    +VFIO Usage Example
    +Assume user wants to access PCI device 0000:06:0d.0
    +$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
    +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
    +# 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,
    + dma_map.size = 1024 * 1024;
    + dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
    + 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()
    +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
    +[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:
    + \-0d.1
    +00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)

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