FreeBSD/Linux Kernel Cross Reference
sys/Documentation/vfio.txt

     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-pci/new_id
   
   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)

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