1VFIO - "Virtual Function I/O"[1]
   3Many modern system now provide DMA and interrupt remapping facilities
   4to help ensure I/O devices behave within the boundaries they've been
   5allotted.  This includes x86 hardware with AMD-Vi and Intel VT-d,
   6POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
   7systems such as Freescale PAMU.  The VFIO driver is an IOMMU/device
   8agnostic framework for exposing direct device access to userspace, in
   9a secure, IOMMU protected environment.  In other words, this allows
  10safe[2], non-privileged, userspace drivers.
  12Why do we want that?  Virtual machines often make use of direct device
  13access ("device assignment") when configured for the highest possible
  14I/O performance.  From a device and host perspective, this simply
  15turns the VM into a userspace driver, with the benefits of
  16significantly reduced latency, higher bandwidth, and direct use of
  17bare-metal device drivers[3].
  19Some applications, particularly in the high performance computing
  20field, also benefit from low-overhead, direct device access from
  21userspace.  Examples include network adapters (often non-TCP/IP based)
  22and compute accelerators.  Prior to VFIO, these drivers had to either
  23go through the full development cycle to become proper upstream
  24driver, be maintained out of tree, or make use of the UIO framework,
  25which has no notion of IOMMU protection, limited interrupt support,
  26and requires root privileges to access things like PCI configuration
  29The VFIO driver framework intends to unify these, replacing both the
  30KVM PCI specific device assignment code as well as provide a more
  31secure, more featureful userspace driver environment than UIO.
  33Groups, Devices, and IOMMUs
  36Devices are the main target of any I/O driver.  Devices typically
  37create a programming interface made up of I/O access, interrupts,
  38and DMA.  Without going into the details of each of these, DMA is
  39by far the most critical aspect for maintaining a secure environment
  40as allowing a device read-write access to system memory imposes the
  41greatest risk to the overall system integrity.
  43To help mitigate this risk, many modern IOMMUs now incorporate
  44isolation properties into what was, in many cases, an interface only
  45meant for translation (ie. solving the addressing problems of devices
  46with limited address spaces).  With this, devices can now be isolated
  47from each other and from arbitrary memory access, thus allowing
  48things like secure direct assignment of devices into virtual machines.
  50This isolation is not always at the granularity of a single device
  51though.  Even when an IOMMU is capable of this, properties of devices,
  52interconnects, and IOMMU topologies can each reduce this isolation.
  53For instance, an individual device may be part of a larger multi-
  54function enclosure.  While the IOMMU may be able to distinguish
  55between devices within the enclosure, the enclosure may not require
  56transactions between devices to reach the IOMMU.  Examples of this
  57could be anything from a multi-function PCI device with backdoors
  58between functions to a non-PCI-ACS (Access Control Services) capable
  59bridge allowing redirection without reaching the IOMMU.  Topology
  60can also play a factor in terms of hiding devices.  A PCIe-to-PCI
  61bridge masks the devices behind it, making transaction appear as if
  62from the bridge itself.  Obviously IOMMU design plays a major factor
  63as well.
  65Therefore, while for the most part an IOMMU may have device level
  66granularity, any system is susceptible to reduced granularity.  The
  67IOMMU API therefore supports a notion of IOMMU groups.  A group is
  68a set of devices which is isolatable from all other devices in the
  69system.  Groups are therefore the unit of ownership used by VFIO.
  71While the group is the minimum granularity that must be used to
  72ensure secure user access, it's not necessarily the preferred
  73granularity.  In IOMMUs which make use of page tables, it may be
  74possible to share a set of page tables between different groups,
  75reducing the overhead both to the platform (reduced TLB thrashing,
  76reduced duplicate page tables), and to the user (programming only
  77a single set of translations).  For this reason, VFIO makes use of
  78a container class, which may hold one or more groups.  A container
  79is created by simply opening the /dev/vfio/vfio character device.
  81On its own, the container provides little functionality, with all
  82but a couple version and extension query interfaces locked away.
  83The user needs to add a group into the container for the next level
  84of functionality.  To do this, the user first needs to identify the
  85group associated with the desired device.  This can be done using
  86the sysfs links described in the example below.  By unbinding the
  87device from the host driver and binding it to a VFIO driver, a new
  88VFIO group will appear for the group as /dev/vfio/$GROUP, where
  89$GROUP is the IOMMU group number of which the device is a member.
  90If the IOMMU group contains multiple devices, each will need to
  91be bound to a VFIO driver before operations on the VFIO group
  92are allowed (it's also sufficient to only unbind the device from
  93host drivers if a VFIO driver is unavailable; this will make the
  94group available, but not that particular device).  TBD - interface
  95for disabling driver probing/locking a device.
  97Once the group is ready, it may be added to the container by opening
  98the VFIO group character device (/dev/vfio/$GROUP) and using the
  99VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
 100previously opened container file.  If desired and if the IOMMU driver
 101supports sharing the IOMMU context between groups, multiple groups may
 102be set to the same container.  If a group fails to set to a container
 103with existing groups, a new empty container will need to be used
 106With a group (or groups) attached to a container, the remaining
 107ioctls become available, enabling access to the VFIO IOMMU interfaces.
 108Additionally, it now becomes possible to get file descriptors for each
 109device within a group using an ioctl on the VFIO group file descriptor.
 111The VFIO device API includes ioctls for describing the device, the I/O
 112regions and their read/write/mmap offsets on the device descriptor, as
 113well as mechanisms for describing and registering interrupt
 116VFIO Usage Example
 119Assume user wants to access PCI device 0000:06:0d.0
 121$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
 124This device is therefore in IOMMU group 26.  This device is on the
 125pci bus, therefore the user will make use of vfio-pci to manage the
 128# modprobe vfio-pci
 130Binding this device to the vfio-pci driver creates the VFIO group
 131character devices for this group:
 133$ lspci -n -s 0000:06:0d.0
 13406:0d.0 0401: 1102:0002 (rev 08)
 135# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
 136# echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
 138Now we need to look at what other devices are in the group to free
 139it for use by VFIO:
 141$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
 142total 0
 143lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
 144        ../../../../devices/pci0000:00/0000:00:1e.0
 145lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
 146        ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
 147lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.1 ->
 148        ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.1
 150This device is behind a PCIe-to-PCI bridge[4], therefore we also
 151need to add device 0000:06:0d.1 to the group following the same
 152procedure as above.  Device 0000:00:1e.0 is a bridge that does
 153not currently have a host driver, therefore it's not required to
 154bind this device to the vfio-pci driver (vfio-pci does not currently
 155support PCI bridges).
 157The final step is to provide the user with access to the group if
 158unprivileged operation is desired (note that /dev/vfio/vfio provides
 159no capabilities on its own and is therefore expected to be set to
 160mode 0666 by the system).
 162# chown user:user /dev/vfio/26
 164The user now has full access to all the devices and the iommu for this
 165group and can access them as follows:
 167        int container, group, device, i;
 168        struct vfio_group_status group_status =
 169                                        { .argsz = sizeof(group_status) };
 170        struct vfio_iommu_x86_info iommu_info = { .argsz = sizeof(iommu_info) };
 171        struct vfio_iommu_x86_dma_map dma_map = { .argsz = sizeof(dma_map) };
 172        struct vfio_device_info device_info = { .argsz = sizeof(device_info) };
 174        /* Create a new container */
 175        container = open("/dev/vfio/vfio, O_RDWR);
 177        if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
 178                /* Unknown API version */
 180        if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_X86_IOMMU))
 181                /* Doesn't support the IOMMU driver we want. */
 183        /* Open the group */
 184        group = open("/dev/vfio/26", O_RDWR);
 186        /* Test the group is viable and available */
 187        ioctl(group, VFIO_GROUP_GET_STATUS, &group_status);
 189        if (!(group_status.flags & VFIO_GROUP_FLAGS_VIABLE))
 190                /* Group is not viable (ie, not all devices bound for vfio) */
 192        /* Add the group to the container */
 193        ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
 195        /* Enable the IOMMU model we want */
 196        ioctl(container, VFIO_SET_IOMMU, VFIO_X86_IOMMU)
 198        /* Get addition IOMMU info */
 199        ioctl(container, VFIO_IOMMU_GET_INFO, &iommu_info);
 201        /* Allocate some space and setup a DMA mapping */
 202        dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
 203                             MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
 204        dma_map.size = 1024 * 1024;
 205        dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
 206        dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
 208        ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
 210        /* Get a file descriptor for the device */
 211        device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
 213        /* Test and setup the device */
 214        ioctl(device, VFIO_DEVICE_GET_INFO, &device_info);
 216        for (i = 0; i < device_info.num_regions; i++) {
 217                struct vfio_region_info reg = { .argsz = sizeof(reg) };
 219                reg.index = i;
 221                ioctl(device, VFIO_DEVICE_GET_REGION_INFO, &reg);
 223                /* Setup mappings... read/write offsets, mmaps
 224                 * For PCI devices, config space is a region */
 225        }
 227        for (i = 0; i < device_info.num_irqs; i++) {
 228                struct vfio_irq_info irq = { .argsz = sizeof(irq) };
 230                irq.index = i;
 232                ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &reg);
 234                /* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
 235        }
 237        /* Gratuitous device reset and go... */
 238        ioctl(device, VFIO_DEVICE_RESET);
 240VFIO User API
 243Please see include/linux/vfio.h for complete API documentation.
 245VFIO bus driver API
 248VFIO bus drivers, such as vfio-pci make use of only a few interfaces
 249into VFIO core.  When devices are bound and unbound to the driver,
 250the driver should call vfio_add_group_dev() and vfio_del_group_dev()
 253extern int vfio_add_group_dev(struct iommu_group *iommu_group,
 254                              struct device *dev,
 255                              const struct vfio_device_ops *ops,
 256                              void *device_data);
 258extern void *vfio_del_group_dev(struct device *dev);
 260vfio_add_group_dev() indicates to the core to begin tracking the
 261specified iommu_group and register the specified dev as owned by
 262a VFIO bus driver.  The driver provides an ops structure for callbacks
 263similar to a file operations structure:
 265struct vfio_device_ops {
 266        int     (*open)(void *device_data);
 267        void    (*release)(void *device_data);
 268        ssize_t (*read)(void *device_data, char __user *buf,
 269                        size_t count, loff_t *ppos);
 270        ssize_t (*write)(void *device_data, const char __user *buf,
 271                         size_t size, loff_t *ppos);
 272        long    (*ioctl)(void *device_data, unsigned int cmd,
 273                         unsigned long arg);
 274        int     (*mmap)(void *device_data, struct vm_area_struct *vma);
 277Each function is passed the device_data that was originally registered
 278in the vfio_add_group_dev() call above.  This allows the bus driver
 279an easy place to store its opaque, private data.  The open/release
 280callbacks are issued when a new file descriptor is created for a
 281device (via VFIO_GROUP_GET_DEVICE_FD).  The ioctl interface provides
 282a direct pass through for VFIO_DEVICE_* ioctls.  The read/write/mmap
 283interfaces implement the device region access defined by the device's
 288[1] VFIO was originally an acronym for "Virtual Function I/O" in its
 289initial implementation by Tom Lyon while as Cisco.  We've since
 290outgrown the acronym, but it's catchy.
 292[2] "safe" also depends upon a device being "well behaved".  It's
 293possible for multi-function devices to have backdoors between
 294functions and even for single function devices to have alternative
 295access to things like PCI config space through MMIO registers.  To
 296guard against the former we can include additional precautions in the
 297IOMMU driver to group multi-function PCI devices together
 298(iommu=group_mf).  The latter we can't prevent, but the IOMMU should
 299still provide isolation.  For PCI, SR-IOV Virtual Functions are the
 300best indicator of "well behaved", as these are designed for
 301virtualization usage models.
 303[3] As always there are trade-offs to virtual machine device
 304assignment that are beyond the scope of VFIO.  It's expected that
 305future IOMMU technologies will reduce some, but maybe not all, of
 306these trade-offs.
 308[4] In this case the device is below a PCI bridge, so transactions
 309from either function of the device are indistinguishable to the iommu:
 312                        \-0d.1
 31400:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)