1==================================
2VFIO - "Virtual Function I/O" [1]_
3==================================
4
5Many modern systems now provide DMA and interrupt remapping facilities
6to help ensure I/O devices behave within the boundaries they've been
7allotted.  This includes x86 hardware with AMD-Vi and Intel VT-d,
8POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
9systems such as Freescale PAMU.  The VFIO driver is an IOMMU/device
10agnostic framework for exposing direct device access to userspace, in
11a secure, IOMMU protected environment.  In other words, this allows
12safe [2]_, non-privileged, userspace drivers.
13
14Why do we want that?  Virtual machines often make use of direct device
15access ("device assignment") when configured for the highest possible
16I/O performance.  From a device and host perspective, this simply
17turns the VM into a userspace driver, with the benefits of
18significantly reduced latency, higher bandwidth, and direct use of
19bare-metal device drivers [3]_.
20
21Some applications, particularly in the high performance computing
22field, also benefit from low-overhead, direct device access from
23userspace.  Examples include network adapters (often non-TCP/IP based)
24and compute accelerators.  Prior to VFIO, these drivers had to either
25go through the full development cycle to become proper upstream
26driver, be maintained out of tree, or make use of the UIO framework,
27which has no notion of IOMMU protection, limited interrupt support,
28and requires root privileges to access things like PCI configuration
29space.
30
31The VFIO driver framework intends to unify these, replacing both the
32KVM PCI specific device assignment code as well as provide a more
33secure, more featureful userspace driver environment than UIO.
34
35Groups, Devices, and IOMMUs
36---------------------------
37
38Devices are the main target of any I/O driver.  Devices typically
39create a programming interface made up of I/O access, interrupts,
40and DMA.  Without going into the details of each of these, DMA is
41by far the most critical aspect for maintaining a secure environment
42as allowing a device read-write access to system memory imposes the
43greatest risk to the overall system integrity.
44
45To help mitigate this risk, many modern IOMMUs now incorporate
46isolation properties into what was, in many cases, an interface only
47meant for translation (ie. solving the addressing problems of devices
48with limited address spaces).  With this, devices can now be isolated
49from each other and from arbitrary memory access, thus allowing
50things like secure direct assignment of devices into virtual machines.
51
52This isolation is not always at the granularity of a single device
53though.  Even when an IOMMU is capable of this, properties of devices,
54interconnects, and IOMMU topologies can each reduce this isolation.
55For instance, an individual device may be part of a larger multi-
56function enclosure.  While the IOMMU may be able to distinguish
57between devices within the enclosure, the enclosure may not require
58transactions between devices to reach the IOMMU.  Examples of this
59could be anything from a multi-function PCI device with backdoors
60between functions to a non-PCI-ACS (Access Control Services) capable
61bridge allowing redirection without reaching the IOMMU.  Topology
62can also play a factor in terms of hiding devices.  A PCIe-to-PCI
63bridge masks the devices behind it, making transaction appear as if
64from the bridge itself.  Obviously IOMMU design plays a major factor
65as well.
66
67Therefore, while for the most part an IOMMU may have device level
68granularity, any system is susceptible to reduced granularity.  The
69IOMMU API therefore supports a notion of IOMMU groups.  A group is
70a set of devices which is isolatable from all other devices in the
71system.  Groups are therefore the unit of ownership used by VFIO.
72
73While the group is the minimum granularity that must be used to
74ensure secure user access, it's not necessarily the preferred
75granularity.  In IOMMUs which make use of page tables, it may be
76possible to share a set of page tables between different groups,
77reducing the overhead both to the platform (reduced TLB thrashing,
78reduced duplicate page tables), and to the user (programming only
79a single set of translations).  For this reason, VFIO makes use of
80a container class, which may hold one or more groups.  A container
81is created by simply opening the /dev/vfio/vfio character device.
82
83On its own, the container provides little functionality, with all
84but a couple version and extension query interfaces locked away.
85The user needs to add a group into the container for the next level
86of functionality.  To do this, the user first needs to identify the
87group associated with the desired device.  This can be done using
88the sysfs links described in the example below.  By unbinding the
89device from the host driver and binding it to a VFIO driver, a new
90VFIO group will appear for the group as /dev/vfio/$GROUP, where
91$GROUP is the IOMMU group number of which the device is a member.
92If the IOMMU group contains multiple devices, each will need to
93be bound to a VFIO driver before operations on the VFIO group
94are allowed (it's also sufficient to only unbind the device from
95host drivers if a VFIO driver is unavailable; this will make the
96group available, but not that particular device).  TBD - interface
97for disabling driver probing/locking a device.
98
99Once the group is ready, it may be added to the container by opening
100the VFIO group character device (/dev/vfio/$GROUP) and using the
101VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
102previously opened container file.  If desired and if the IOMMU driver
103supports sharing the IOMMU context between groups, multiple groups may
104be set to the same container.  If a group fails to set to a container
105with existing groups, a new empty container will need to be used
106instead.
107
108With a group (or groups) attached to a container, the remaining
109ioctls become available, enabling access to the VFIO IOMMU interfaces.
110Additionally, it now becomes possible to get file descriptors for each
111device within a group using an ioctl on the VFIO group file descriptor.
112
113The VFIO device API includes ioctls for describing the device, the I/O
114regions and their read/write/mmap offsets on the device descriptor, as
115well as mechanisms for describing and registering interrupt
116notifications.
117
118VFIO Usage Example
119------------------
120
121Assume user wants to access PCI device 0000:06:0d.0::
122
123	$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
124	../../../../kernel/iommu_groups/26
125
126This device is therefore in IOMMU group 26.  This device is on the
127pci bus, therefore the user will make use of vfio-pci to manage the
128group::
129
130	# modprobe vfio-pci
131
132Binding this device to the vfio-pci driver creates the VFIO group
133character devices for this group::
134
135	$ lspci -n -s 0000:06:0d.0
136	06:0d.0 0401: 1102:0002 (rev 08)
137	# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
138	# echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
139
140Now we need to look at what other devices are in the group to free
141it for use by VFIO::
142
143	$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
144	total 0
145	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
146		../../../../devices/pci0000:00/0000:00:1e.0
147	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
148		../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
149	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.1 ->
150		../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.1
151
152This device is behind a PCIe-to-PCI bridge [4]_, therefore we also
153need to add device 0000:06:0d.1 to the group following the same
154procedure as above.  Device 0000:00:1e.0 is a bridge that does
155not currently have a host driver, therefore it's not required to
156bind this device to the vfio-pci driver (vfio-pci does not currently
157support PCI bridges).
158
159The final step is to provide the user with access to the group if
160unprivileged operation is desired (note that /dev/vfio/vfio provides
161no capabilities on its own and is therefore expected to be set to
162mode 0666 by the system)::
163
164	# chown user:user /dev/vfio/26
165
166The user now has full access to all the devices and the iommu for this
167group and can access them as follows::
168
169	int container, group, device, i;
170	struct vfio_group_status group_status =
171					{ .argsz = sizeof(group_status) };
172	struct vfio_iommu_type1_info iommu_info = { .argsz = sizeof(iommu_info) };
173	struct vfio_iommu_type1_dma_map dma_map = { .argsz = sizeof(dma_map) };
174	struct vfio_device_info device_info = { .argsz = sizeof(device_info) };
175
176	/* Create a new container */
177	container = open("/dev/vfio/vfio", O_RDWR);
178
179	if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
180		/* Unknown API version */
181
182	if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_TYPE1_IOMMU))
183		/* Doesn't support the IOMMU driver we want. */
184
185	/* Open the group */
186	group = open("/dev/vfio/26", O_RDWR);
187
188	/* Test the group is viable and available */
189	ioctl(group, VFIO_GROUP_GET_STATUS, &group_status);
190
191	if (!(group_status.flags & VFIO_GROUP_FLAGS_VIABLE))
192		/* Group is not viable (ie, not all devices bound for vfio) */
193
194	/* Add the group to the container */
195	ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
196
197	/* Enable the IOMMU model we want */
198	ioctl(container, VFIO_SET_IOMMU, VFIO_TYPE1_IOMMU);
199
200	/* Get addition IOMMU info */
201	ioctl(container, VFIO_IOMMU_GET_INFO, &iommu_info);
202
203	/* Allocate some space and setup a DMA mapping */
204	dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
205			     MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
206	dma_map.size = 1024 * 1024;
207	dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
208	dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
209
210	ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
211
212	/* Get a file descriptor for the device */
213	device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
214
215	/* Test and setup the device */
216	ioctl(device, VFIO_DEVICE_GET_INFO, &device_info);
217
218	for (i = 0; i < device_info.num_regions; i++) {
219		struct vfio_region_info reg = { .argsz = sizeof(reg) };
220
221		reg.index = i;
222
223		ioctl(device, VFIO_DEVICE_GET_REGION_INFO, &reg);
224
225		/* Setup mappings... read/write offsets, mmaps
226		 * For PCI devices, config space is a region */
227	}
228
229	for (i = 0; i < device_info.num_irqs; i++) {
230		struct vfio_irq_info irq = { .argsz = sizeof(irq) };
231
232		irq.index = i;
233
234		ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &irq);
235
236		/* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
237	}
238
239	/* Gratuitous device reset and go... */
240	ioctl(device, VFIO_DEVICE_RESET);
241
242VFIO User API
243-------------------------------------------------------------------------------
244
245Please see include/linux/vfio.h for complete API documentation.
246
247VFIO bus driver API
248-------------------------------------------------------------------------------
249
250VFIO bus drivers, such as vfio-pci make use of only a few interfaces
251into VFIO core.  When devices are bound and unbound to the driver,
252Following interfaces are called when devices are bound to and
253unbound from the driver::
254
255	int vfio_register_group_dev(struct vfio_device *device);
256	int vfio_register_emulated_iommu_dev(struct vfio_device *device);
257	void vfio_unregister_group_dev(struct vfio_device *device);
258
259The driver should embed the vfio_device in its own structure and use
260vfio_alloc_device() to allocate the structure, and can register
261@init/@release callbacks to manage any private state wrapping the
262vfio_device::
263
264	vfio_alloc_device(dev_struct, member, dev, ops);
265	void vfio_put_device(struct vfio_device *device);
266
267vfio_register_group_dev() indicates to the core to begin tracking the
268iommu_group of the specified dev and register the dev as owned by a VFIO bus
269driver. Once vfio_register_group_dev() returns it is possible for userspace to
270start accessing the driver, thus the driver should ensure it is completely
271ready before calling it. The driver provides an ops structure for callbacks
272similar to a file operations structure::
273
274	struct vfio_device_ops {
275		char	*name;
276		int	(*init)(struct vfio_device *vdev);
277		void	(*release)(struct vfio_device *vdev);
278		int	(*bind_iommufd)(struct vfio_device *vdev,
279					struct iommufd_ctx *ictx, u32 *out_device_id);
280		void	(*unbind_iommufd)(struct vfio_device *vdev);
281		int	(*attach_ioas)(struct vfio_device *vdev, u32 *pt_id);
282		int	(*open_device)(struct vfio_device *vdev);
283		void	(*close_device)(struct vfio_device *vdev);
284		ssize_t	(*read)(struct vfio_device *vdev, char __user *buf,
285				size_t count, loff_t *ppos);
286		ssize_t	(*write)(struct vfio_device *vdev, const char __user *buf,
287			 size_t count, loff_t *size);
288		long	(*ioctl)(struct vfio_device *vdev, unsigned int cmd,
289				 unsigned long arg);
290		int	(*mmap)(struct vfio_device *vdev, struct vm_area_struct *vma);
291		void	(*request)(struct vfio_device *vdev, unsigned int count);
292		int	(*match)(struct vfio_device *vdev, char *buf);
293		void	(*dma_unmap)(struct vfio_device *vdev, u64 iova, u64 length);
294		int	(*device_feature)(struct vfio_device *device, u32 flags,
295					  void __user *arg, size_t argsz);
296	};
297
298Each function is passed the vdev that was originally registered
299in the vfio_register_group_dev() or vfio_register_emulated_iommu_dev()
300call above. This allows the bus driver to obtain its private data using
301container_of().
302
303::
304
305	- The init/release callbacks are issued when vfio_device is initialized
306	  and released.
307
308	- The open/close device callbacks are issued when the first
309	  instance of a file descriptor for the device is created (eg.
310	  via VFIO_GROUP_GET_DEVICE_FD) for a user session.
311
312	- The ioctl callback provides a direct pass through for some VFIO_DEVICE_*
313	  ioctls.
314
315	- The [un]bind_iommufd callbacks are issued when the device is bound to
316	  and unbound from iommufd.
317
318	- The attach_ioas callback is issued when the device is attached to an
319	  IOAS managed by the bound iommufd. The attached IOAS is automatically
320	  detached when the device is unbound from iommufd.
321
322	- The read/write/mmap callbacks implement the device region access defined
323	  by the device's own VFIO_DEVICE_GET_REGION_INFO ioctl.
324
325	- The request callback is issued when device is going to be unregistered,
326	  such as when trying to unbind the device from the vfio bus driver.
327
328	- The dma_unmap callback is issued when a range of iovas are unmapped
329	  in the container or IOAS attached by the device. Drivers which make
330	  use of the vfio page pinning interface must implement this callback in
331	  order to unpin pages within the dma_unmap range. Drivers must tolerate
332	  this callback even before calls to open_device().
333
334PPC64 sPAPR implementation note
335-------------------------------
336
337This implementation has some specifics:
338
3391) On older systems (POWER7 with P5IOC2/IODA1) only one IOMMU group per
340   container is supported as an IOMMU table is allocated at the boot time,
341   one table per a IOMMU group which is a Partitionable Endpoint (PE)
342   (PE is often a PCI domain but not always).
343
344   Newer systems (POWER8 with IODA2) have improved hardware design which allows
345   to remove this limitation and have multiple IOMMU groups per a VFIO
346   container.
347
3482) The hardware supports so called DMA windows - the PCI address range
349   within which DMA transfer is allowed, any attempt to access address space
350   out of the window leads to the whole PE isolation.
351
3523) PPC64 guests are paravirtualized but not fully emulated. There is an API
353   to map/unmap pages for DMA, and it normally maps 1..32 pages per call and
354   currently there is no way to reduce the number of calls. In order to make
355   things faster, the map/unmap handling has been implemented in real mode
356   which provides an excellent performance which has limitations such as
357   inability to do locked pages accounting in real time.
358
3594) According to sPAPR specification, A Partitionable Endpoint (PE) is an I/O
360   subtree that can be treated as a unit for the purposes of partitioning and
361   error recovery. A PE may be a single or multi-function IOA (IO Adapter), a
362   function of a multi-function IOA, or multiple IOAs (possibly including
363   switch and bridge structures above the multiple IOAs). PPC64 guests detect
364   PCI errors and recover from them via EEH RTAS services, which works on the
365   basis of additional ioctl commands.
366
367   So 4 additional ioctls have been added:
368
369	VFIO_IOMMU_SPAPR_TCE_GET_INFO
370		returns the size and the start of the DMA window on the PCI bus.
371
372	VFIO_IOMMU_ENABLE
373		enables the container. The locked pages accounting
374		is done at this point. This lets user first to know what
375		the DMA window is and adjust rlimit before doing any real job.
376
377	VFIO_IOMMU_DISABLE
378		disables the container.
379
380	VFIO_EEH_PE_OP
381		provides an API for EEH setup, error detection and recovery.
382
383   The code flow from the example above should be slightly changed::
384
385	struct vfio_eeh_pe_op pe_op = { .argsz = sizeof(pe_op), .flags = 0 };
386
387	.....
388	/* Add the group to the container */
389	ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
390
391	/* Enable the IOMMU model we want */
392	ioctl(container, VFIO_SET_IOMMU, VFIO_SPAPR_TCE_IOMMU)
393
394	/* Get addition sPAPR IOMMU info */
395	vfio_iommu_spapr_tce_info spapr_iommu_info;
396	ioctl(container, VFIO_IOMMU_SPAPR_TCE_GET_INFO, &spapr_iommu_info);
397
398	if (ioctl(container, VFIO_IOMMU_ENABLE))
399		/* Cannot enable container, may be low rlimit */
400
401	/* Allocate some space and setup a DMA mapping */
402	dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
403			     MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
404
405	dma_map.size = 1024 * 1024;
406	dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
407	dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
408
409	/* Check here is .iova/.size are within DMA window from spapr_iommu_info */
410	ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
411
412	/* Get a file descriptor for the device */
413	device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
414
415	....
416
417	/* Gratuitous device reset and go... */
418	ioctl(device, VFIO_DEVICE_RESET);
419
420	/* Make sure EEH is supported */
421	ioctl(container, VFIO_CHECK_EXTENSION, VFIO_EEH);
422
423	/* Enable the EEH functionality on the device */
424	pe_op.op = VFIO_EEH_PE_ENABLE;
425	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
426
427	/* You're suggested to create additional data struct to represent
428	 * PE, and put child devices belonging to same IOMMU group to the
429	 * PE instance for later reference.
430	 */
431
432	/* Check the PE's state and make sure it's in functional state */
433	pe_op.op = VFIO_EEH_PE_GET_STATE;
434	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
435
436	/* Save device state using pci_save_state().
437	 * EEH should be enabled on the specified device.
438	 */
439
440	....
441
442	/* Inject EEH error, which is expected to be caused by 32-bits
443	 * config load.
444	 */
445	pe_op.op = VFIO_EEH_PE_INJECT_ERR;
446	pe_op.err.type = EEH_ERR_TYPE_32;
447	pe_op.err.func = EEH_ERR_FUNC_LD_CFG_ADDR;
448	pe_op.err.addr = 0ul;
449	pe_op.err.mask = 0ul;
450	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
451
452	....
453
454	/* When 0xFF's returned from reading PCI config space or IO BARs
455	 * of the PCI device. Check the PE's state to see if that has been
456	 * frozen.
457	 */
458	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
459
460	/* Waiting for pending PCI transactions to be completed and don't
461	 * produce any more PCI traffic from/to the affected PE until
462	 * recovery is finished.
463	 */
464
465	/* Enable IO for the affected PE and collect logs. Usually, the
466	 * standard part of PCI config space, AER registers are dumped
467	 * as logs for further analysis.
468	 */
469	pe_op.op = VFIO_EEH_PE_UNFREEZE_IO;
470	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
471
472	/*
473	 * Issue PE reset: hot or fundamental reset. Usually, hot reset
474	 * is enough. However, the firmware of some PCI adapters would
475	 * require fundamental reset.
476	 */
477	pe_op.op = VFIO_EEH_PE_RESET_HOT;
478	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
479	pe_op.op = VFIO_EEH_PE_RESET_DEACTIVATE;
480	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
481
482	/* Configure the PCI bridges for the affected PE */
483	pe_op.op = VFIO_EEH_PE_CONFIGURE;
484	ioctl(container, VFIO_EEH_PE_OP, &pe_op);
485
486	/* Restored state we saved at initialization time. pci_restore_state()
487	 * is good enough as an example.
488	 */
489
490	/* Hopefully, error is recovered successfully. Now, you can resume to
491	 * start PCI traffic to/from the affected PE.
492	 */
493
494	....
495
4965) There is v2 of SPAPR TCE IOMMU. It deprecates VFIO_IOMMU_ENABLE/
497   VFIO_IOMMU_DISABLE and implements 2 new ioctls:
498   VFIO_IOMMU_SPAPR_REGISTER_MEMORY and VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY
499   (which are unsupported in v1 IOMMU).
500
501   PPC64 paravirtualized guests generate a lot of map/unmap requests,
502   and the handling of those includes pinning/unpinning pages and updating
503   mm::locked_vm counter to make sure we do not exceed the rlimit.
504   The v2 IOMMU splits accounting and pinning into separate operations:
505
506   - VFIO_IOMMU_SPAPR_REGISTER_MEMORY/VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY ioctls
507     receive a user space address and size of the block to be pinned.
508     Bisecting is not supported and VFIO_IOMMU_UNREGISTER_MEMORY is expected to
509     be called with the exact address and size used for registering
510     the memory block. The userspace is not expected to call these often.
511     The ranges are stored in a linked list in a VFIO container.
512
513   - VFIO_IOMMU_MAP_DMA/VFIO_IOMMU_UNMAP_DMA ioctls only update the actual
514     IOMMU table and do not do pinning; instead these check that the userspace
515     address is from pre-registered range.
516
517   This separation helps in optimizing DMA for guests.
518
5196) sPAPR specification allows guests to have an additional DMA window(s) on
520   a PCI bus with a variable page size. Two ioctls have been added to support
521   this: VFIO_IOMMU_SPAPR_TCE_CREATE and VFIO_IOMMU_SPAPR_TCE_REMOVE.
522   The platform has to support the functionality or error will be returned to
523   the userspace. The existing hardware supports up to 2 DMA windows, one is
524   2GB long, uses 4K pages and called "default 32bit window"; the other can
525   be as big as entire RAM, use different page size, it is optional - guests
526   create those in run-time if the guest driver supports 64bit DMA.
527
528   VFIO_IOMMU_SPAPR_TCE_CREATE receives a page shift, a DMA window size and
529   a number of TCE table levels (if a TCE table is going to be big enough and
530   the kernel may not be able to allocate enough of physically contiguous
531   memory). It creates a new window in the available slot and returns the bus
532   address where the new window starts. Due to hardware limitation, the user
533   space cannot choose the location of DMA windows.
534
535   VFIO_IOMMU_SPAPR_TCE_REMOVE receives the bus start address of the window
536   and removes it.
537
538-------------------------------------------------------------------------------
539
540.. [1] VFIO was originally an acronym for "Virtual Function I/O" in its
541   initial implementation by Tom Lyon while as Cisco.  We've since
542   outgrown the acronym, but it's catchy.
543
544.. [2] "safe" also depends upon a device being "well behaved".  It's
545   possible for multi-function devices to have backdoors between
546   functions and even for single function devices to have alternative
547   access to things like PCI config space through MMIO registers.  To
548   guard against the former we can include additional precautions in the
549   IOMMU driver to group multi-function PCI devices together
550   (iommu=group_mf).  The latter we can't prevent, but the IOMMU should
551   still provide isolation.  For PCI, SR-IOV Virtual Functions are the
552   best indicator of "well behaved", as these are designed for
553   virtualization usage models.
554
555.. [3] As always there are trade-offs to virtual machine device
556   assignment that are beyond the scope of VFIO.  It's expected that
557   future IOMMU technologies will reduce some, but maybe not all, of
558   these trade-offs.
559
560.. [4] In this case the device is below a PCI bridge, so transactions
561   from either function of the device are indistinguishable to the iommu::
562
563	-[0000:00]-+-1e.0-[06]--+-0d.0
564				\-0d.1
565
566	00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
567