Documentation/vfio.txt - kernel/msm-5.4 - Gitiles

 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_TYPE1_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_TYPE1_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)
	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_TYPE1_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_TYPE1_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)