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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-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 ->
lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
lrwxrwxrwx. 1 root root 0 Apr 23 16:13 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);
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:
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
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