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