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Based on kernel version 3.9. Page generated on 2013-05-02 23:15 EST.

1	VFIO - "Virtual Function I/O"[1]
2	-------------------------------------------------------------------------------
3	Many modern system now provide DMA and interrupt remapping facilities
4	to help ensure I/O devices behave within the boundaries they've been
5	allotted.  This includes x86 hardware with AMD-Vi and Intel VT-d,
6	POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
7	systems such as Freescale PAMU.  The VFIO driver is an IOMMU/device
8	agnostic framework for exposing direct device access to userspace, in
9	a secure, IOMMU protected environment.  In other words, this allows
10	safe[2], non-privileged, userspace drivers.
11	
12	Why do we want that?  Virtual machines often make use of direct device
13	access ("device assignment") when configured for the highest possible
14	I/O performance.  From a device and host perspective, this simply
15	turns the VM into a userspace driver, with the benefits of
16	significantly reduced latency, higher bandwidth, and direct use of
17	bare-metal device drivers[3].
18	
19	Some applications, particularly in the high performance computing
20	field, also benefit from low-overhead, direct device access from
21	userspace.  Examples include network adapters (often non-TCP/IP based)
22	and compute accelerators.  Prior to VFIO, these drivers had to either
23	go through the full development cycle to become proper upstream
24	driver, be maintained out of tree, or make use of the UIO framework,
25	which has no notion of IOMMU protection, limited interrupt support,
26	and requires root privileges to access things like PCI configuration
27	space.
28	
29	The VFIO driver framework intends to unify these, replacing both the
30	KVM PCI specific device assignment code as well as provide a more
31	secure, more featureful userspace driver environment than UIO.
32	
33	Groups, Devices, and IOMMUs
34	-------------------------------------------------------------------------------
35	
36	Devices are the main target of any I/O driver.  Devices typically
37	create a programming interface made up of I/O access, interrupts,
38	and DMA.  Without going into the details of each of these, DMA is
39	by far the most critical aspect for maintaining a secure environment
40	as allowing a device read-write access to system memory imposes the
41	greatest risk to the overall system integrity.
42	
43	To help mitigate this risk, many modern IOMMUs now incorporate
44	isolation properties into what was, in many cases, an interface only
45	meant for translation (ie. solving the addressing problems of devices
46	with limited address spaces).  With this, devices can now be isolated
47	from each other and from arbitrary memory access, thus allowing
48	things like secure direct assignment of devices into virtual machines.
49	
50	This isolation is not always at the granularity of a single device
51	though.  Even when an IOMMU is capable of this, properties of devices,
52	interconnects, and IOMMU topologies can each reduce this isolation.
53	For instance, an individual device may be part of a larger multi-
54	function enclosure.  While the IOMMU may be able to distinguish
55	between devices within the enclosure, the enclosure may not require
56	transactions between devices to reach the IOMMU.  Examples of this
57	could be anything from a multi-function PCI device with backdoors
58	between functions to a non-PCI-ACS (Access Control Services) capable
59	bridge allowing redirection without reaching the IOMMU.  Topology
60	can also play a factor in terms of hiding devices.  A PCIe-to-PCI
61	bridge masks the devices behind it, making transaction appear as if
62	from the bridge itself.  Obviously IOMMU design plays a major factor
63	as well.
64	
65	Therefore, while for the most part an IOMMU may have device level
66	granularity, any system is susceptible to reduced granularity.  The
67	IOMMU API therefore supports a notion of IOMMU groups.  A group is
68	a set of devices which is isolatable from all other devices in the
69	system.  Groups are therefore the unit of ownership used by VFIO.
70	
71	While the group is the minimum granularity that must be used to
72	ensure secure user access, it's not necessarily the preferred
73	granularity.  In IOMMUs which make use of page tables, it may be
74	possible to share a set of page tables between different groups,
75	reducing the overhead both to the platform (reduced TLB thrashing,
76	reduced duplicate page tables), and to the user (programming only
77	a single set of translations).  For this reason, VFIO makes use of
78	a container class, which may hold one or more groups.  A container
79	is created by simply opening the /dev/vfio/vfio character device.
80	
81	On its own, the container provides little functionality, with all
82	but a couple version and extension query interfaces locked away.
83	The user needs to add a group into the container for the next level
84	of functionality.  To do this, the user first needs to identify the
85	group associated with the desired device.  This can be done using
86	the sysfs links described in the example below.  By unbinding the
87	device from the host driver and binding it to a VFIO driver, a new
88	VFIO group will appear for the group as /dev/vfio/$GROUP, where
89	$GROUP is the IOMMU group number of which the device is a member.
90	If the IOMMU group contains multiple devices, each will need to
91	be bound to a VFIO driver before operations on the VFIO group
92	are allowed (it's also sufficient to only unbind the device from
93	host drivers if a VFIO driver is unavailable; this will make the
94	group available, but not that particular device).  TBD - interface
95	for disabling driver probing/locking a device.
96	
97	Once the group is ready, it may be added to the container by opening
98	the VFIO group character device (/dev/vfio/$GROUP) and using the
99	VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
100	previously opened container file.  If desired and if the IOMMU driver
101	supports sharing the IOMMU context between groups, multiple groups may
102	be set to the same container.  If a group fails to set to a container
103	with existing groups, a new empty container will need to be used
104	instead.
105	
106	With a group (or groups) attached to a container, the remaining
107	ioctls become available, enabling access to the VFIO IOMMU interfaces.
108	Additionally, it now becomes possible to get file descriptors for each
109	device within a group using an ioctl on the VFIO group file descriptor.
110	
111	The VFIO device API includes ioctls for describing the device, the I/O
112	regions and their read/write/mmap offsets on the device descriptor, as
113	well as mechanisms for describing and registering interrupt
114	notifications.
115	
116	VFIO Usage Example
117	-------------------------------------------------------------------------------
118	
119	Assume user wants to access PCI device 0000:06:0d.0
120	
121	$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
122	../../../../kernel/iommu_groups/26
123	
124	This device is therefore in IOMMU group 26.  This device is on the
125	pci bus, therefore the user will make use of vfio-pci to manage the
126	group:
127	
128	# modprobe vfio-pci
129	
130	Binding this device to the vfio-pci driver creates the VFIO group
131	character devices for this group:
132	
133	$ lspci -n -s 0000:06:0d.0
134	06:0d.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
137	
138	Now we need to look at what other devices are in the group to free
139	it for use by VFIO:
140	
141	$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
142	total 0
143	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
144		../../../../devices/pci0000:00/0000:00:1e.0
145	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
146		../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
147	lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.1 ->
148		../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.1
149	
150	This device is behind a PCIe-to-PCI bridge[4], therefore we also
151	need to add device 0000:06:0d.1 to the group following the same
152	procedure as above.  Device 0000:00:1e.0 is a bridge that does
153	not currently have a host driver, therefore it's not required to
154	bind this device to the vfio-pci driver (vfio-pci does not currently
155	support PCI bridges).
156	
157	The final step is to provide the user with access to the group if
158	unprivileged operation is desired (note that /dev/vfio/vfio provides
159	no capabilities on its own and is therefore expected to be set to
160	mode 0666 by the system).
161	
162	# chown user:user /dev/vfio/26
163	
164	The user now has full access to all the devices and the iommu for this
165	group and can access them as follows:
166	
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) };
173	
174		/* Create a new container */
175		container = open("/dev/vfio/vfio, O_RDWR);
176	
177		if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
178			/* Unknown API version */
179	
180		if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_X86_IOMMU))
181			/* Doesn't support the IOMMU driver we want. */
182	
183		/* Open the group */
184		group = open("/dev/vfio/26", O_RDWR);
185	
186		/* Test the group is viable and available */
187		ioctl(group, VFIO_GROUP_GET_STATUS, &group_status);
188	
189		if (!(group_status.flags & VFIO_GROUP_FLAGS_VIABLE))
190			/* Group is not viable (ie, not all devices bound for vfio) */
191	
192		/* Add the group to the container */
193		ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
194	
195		/* Enable the IOMMU model we want */
196		ioctl(container, VFIO_SET_IOMMU, VFIO_X86_IOMMU)
197	
198		/* Get addition IOMMU info */
199		ioctl(container, VFIO_IOMMU_GET_INFO, &iommu_info);
200	
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;
207	
208		ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
209	
210		/* Get a file descriptor for the device */
211		device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
212	
213		/* Test and setup the device */
214		ioctl(device, VFIO_DEVICE_GET_INFO, &device_info);
215	
216		for (i = 0; i < device_info.num_regions; i++) {
217			struct vfio_region_info reg = { .argsz = sizeof(reg) };
218	
219			reg.index = i;
220	
221			ioctl(device, VFIO_DEVICE_GET_REGION_INFO, &reg);
222	
223			/* Setup mappings... read/write offsets, mmaps
224			 * For PCI devices, config space is a region */
225		}
226	
227		for (i = 0; i < device_info.num_irqs; i++) {
228			struct vfio_irq_info irq = { .argsz = sizeof(irq) };
229	
230			irq.index = i;
231	
232			ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &reg);
233	
234			/* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
235		}
236	
237		/* Gratuitous device reset and go... */
238		ioctl(device, VFIO_DEVICE_RESET);
239	
240	VFIO User API
241	-------------------------------------------------------------------------------
242	
243	Please see include/linux/vfio.h for complete API documentation.
244	
245	VFIO bus driver API
246	-------------------------------------------------------------------------------
247	
248	VFIO bus drivers, such as vfio-pci make use of only a few interfaces
249	into VFIO core.  When devices are bound and unbound to the driver,
250	the driver should call vfio_add_group_dev() and vfio_del_group_dev()
251	respectively:
252	
253	extern 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);
257	
258	extern void *vfio_del_group_dev(struct device *dev);
259	
260	vfio_add_group_dev() indicates to the core to begin tracking the
261	specified iommu_group and register the specified dev as owned by
262	a VFIO bus driver.  The driver provides an ops structure for callbacks
263	similar to a file operations structure:
264	
265	struct 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);
275	};
276	
277	Each function is passed the device_data that was originally registered
278	in the vfio_add_group_dev() call above.  This allows the bus driver
279	an easy place to store its opaque, private data.  The open/release
280	callbacks are issued when a new file descriptor is created for a
281	device (via VFIO_GROUP_GET_DEVICE_FD).  The ioctl interface provides
282	a direct pass through for VFIO_DEVICE_* ioctls.  The read/write/mmap
283	interfaces implement the device region access defined by the device's
284	own VFIO_DEVICE_GET_REGION_INFO ioctl.
285	
286	-------------------------------------------------------------------------------
287	
288	[1] VFIO was originally an acronym for "Virtual Function I/O" in its
289	initial implementation by Tom Lyon while as Cisco.  We've since
290	outgrown the acronym, but it's catchy.
291	
292	[2] "safe" also depends upon a device being "well behaved".  It's
293	possible for multi-function devices to have backdoors between
294	functions and even for single function devices to have alternative
295	access to things like PCI config space through MMIO registers.  To
296	guard against the former we can include additional precautions in the
297	IOMMU driver to group multi-function PCI devices together
298	(iommu=group_mf).  The latter we can't prevent, but the IOMMU should
299	still provide isolation.  For PCI, SR-IOV Virtual Functions are the
300	best indicator of "well behaved", as these are designed for
301	virtualization usage models.
302	
303	[3] As always there are trade-offs to virtual machine device
304	assignment that are beyond the scope of VFIO.  It's expected that
305	future IOMMU technologies will reduce some, but maybe not all, of
306	these trade-offs.
307	
308	[4] In this case the device is below a PCI bridge, so transactions
309	from either function of the device are indistinguishable to the iommu:
310	
311	-[0000:00]-+-1e.0-[06]--+-0d.0
312	                        \-0d.1
313	
314	00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
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