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Alex Williamson4a5b2a22012-07-31 08:16:23 -06001VFIO - "Virtual Function I/O"[1]
2-------------------------------------------------------------------------------
3Many modern system now provide DMA and interrupt remapping facilities
4to help ensure I/O devices behave within the boundaries they've been
5allotted. This includes x86 hardware with AMD-Vi and Intel VT-d,
6POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
7systems such as Freescale PAMU. The VFIO driver is an IOMMU/device
8agnostic framework for exposing direct device access to userspace, in
9a secure, IOMMU protected environment. In other words, this allows
10safe[2], non-privileged, userspace drivers.
11
12Why do we want that? Virtual machines often make use of direct device
13access ("device assignment") when configured for the highest possible
14I/O performance. From a device and host perspective, this simply
15turns the VM into a userspace driver, with the benefits of
16significantly reduced latency, higher bandwidth, and direct use of
17bare-metal device drivers[3].
18
19Some applications, particularly in the high performance computing
20field, also benefit from low-overhead, direct device access from
21userspace. Examples include network adapters (often non-TCP/IP based)
22and compute accelerators. Prior to VFIO, these drivers had to either
23go through the full development cycle to become proper upstream
24driver, be maintained out of tree, or make use of the UIO framework,
25which has no notion of IOMMU protection, limited interrupt support,
26and requires root privileges to access things like PCI configuration
27space.
28
29The VFIO driver framework intends to unify these, replacing both the
30KVM PCI specific device assignment code as well as provide a more
31secure, more featureful userspace driver environment than UIO.
32
33Groups, Devices, and IOMMUs
34-------------------------------------------------------------------------------
35
36Devices are the main target of any I/O driver. Devices typically
37create a programming interface made up of I/O access, interrupts,
38and DMA. Without going into the details of each of these, DMA is
39by far the most critical aspect for maintaining a secure environment
40as allowing a device read-write access to system memory imposes the
41greatest risk to the overall system integrity.
42
43To help mitigate this risk, many modern IOMMUs now incorporate
44isolation properties into what was, in many cases, an interface only
45meant for translation (ie. solving the addressing problems of devices
46with limited address spaces). With this, devices can now be isolated
47from each other and from arbitrary memory access, thus allowing
48things like secure direct assignment of devices into virtual machines.
49
50This isolation is not always at the granularity of a single device
51though. Even when an IOMMU is capable of this, properties of devices,
52interconnects, and IOMMU topologies can each reduce this isolation.
53For instance, an individual device may be part of a larger multi-
54function enclosure. While the IOMMU may be able to distinguish
55between devices within the enclosure, the enclosure may not require
56transactions between devices to reach the IOMMU. Examples of this
57could be anything from a multi-function PCI device with backdoors
58between functions to a non-PCI-ACS (Access Control Services) capable
59bridge allowing redirection without reaching the IOMMU. Topology
60can also play a factor in terms of hiding devices. A PCIe-to-PCI
61bridge masks the devices behind it, making transaction appear as if
62from the bridge itself. Obviously IOMMU design plays a major factor
63as well.
64
65Therefore, while for the most part an IOMMU may have device level
66granularity, any system is susceptible to reduced granularity. The
67IOMMU API therefore supports a notion of IOMMU groups. A group is
68a set of devices which is isolatable from all other devices in the
69system. Groups are therefore the unit of ownership used by VFIO.
70
71While the group is the minimum granularity that must be used to
72ensure secure user access, it's not necessarily the preferred
73granularity. In IOMMUs which make use of page tables, it may be
74possible to share a set of page tables between different groups,
75reducing the overhead both to the platform (reduced TLB thrashing,
76reduced duplicate page tables), and to the user (programming only
77a single set of translations). For this reason, VFIO makes use of
78a container class, which may hold one or more groups. A container
79is created by simply opening the /dev/vfio/vfio character device.
80
81On its own, the container provides little functionality, with all
82but a couple version and extension query interfaces locked away.
83The user needs to add a group into the container for the next level
84of functionality. To do this, the user first needs to identify the
85group associated with the desired device. This can be done using
86the sysfs links described in the example below. By unbinding the
87device from the host driver and binding it to a VFIO driver, a new
88VFIO 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.
90If the IOMMU group contains multiple devices, each will need to
91be bound to a VFIO driver before operations on the VFIO group
92are allowed (it's also sufficient to only unbind the device from
93host drivers if a VFIO driver is unavailable; this will make the
94group available, but not that particular device). TBD - interface
95for disabling driver probing/locking a device.
96
97Once the group is ready, it may be added to the container by opening
98the VFIO group character device (/dev/vfio/$GROUP) and using the
99VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
100previously opened container file. If desired and if the IOMMU driver
101supports sharing the IOMMU context between groups, multiple groups may
102be set to the same container. If a group fails to set to a container
103with existing groups, a new empty container will need to be used
104instead.
105
106With a group (or groups) attached to a container, the remaining
107ioctls become available, enabling access to the VFIO IOMMU interfaces.
108Additionally, it now becomes possible to get file descriptors for each
109device within a group using an ioctl on the VFIO group file descriptor.
110
111The VFIO device API includes ioctls for describing the device, the I/O
112regions and their read/write/mmap offsets on the device descriptor, as
113well as mechanisms for describing and registering interrupt
114notifications.
115
116VFIO Usage Example
117-------------------------------------------------------------------------------
118
119Assume user wants to access PCI device 0000:06:0d.0
120
121$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
122../../../../kernel/iommu_groups/26
123
124This device is therefore in IOMMU group 26. This device is on the
125pci bus, therefore the user will make use of vfio-pci to manage the
126group:
127
128# modprobe vfio-pci
129
130Binding this device to the vfio-pci driver creates the VFIO group
131character devices for this group:
132
133$ lspci -n -s 0000:06:0d.0
13406:0d.0 0401: 1102:0002 (rev 08)
135# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
Alex Williamsonb37b5932012-09-21 10:48:03 -0600136# echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600137
138Now we need to look at what other devices are in the group to free
139it for use by VFIO:
140
141$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
142total 0
143lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
144 ../../../../devices/pci0000:00/0000:00:1e.0
145lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
146 ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
147lrwxrwxrwx. 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
150This device is behind a PCIe-to-PCI bridge[4], therefore we also
151need to add device 0000:06:0d.1 to the group following the same
152procedure as above. Device 0000:00:1e.0 is a bridge that does
153not currently have a host driver, therefore it's not required to
154bind this device to the vfio-pci driver (vfio-pci does not currently
155support PCI bridges).
156
157The final step is to provide the user with access to the group if
158unprivileged operation is desired (note that /dev/vfio/vfio provides
159no capabilities on its own and is therefore expected to be set to
160mode 0666 by the system).
161
162# chown user:user /dev/vfio/26
163
164The user now has full access to all the devices and the iommu for this
165group 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) };
Zi Shen Limdac09b52013-09-05 16:36:21 -0600170 struct vfio_iommu_type1_info iommu_info = { .argsz = sizeof(iommu_info) };
171 struct vfio_iommu_type1_dma_map dma_map = { .argsz = sizeof(dma_map) };
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600172 struct vfio_device_info device_info = { .argsz = sizeof(device_info) };
173
174 /* Create a new container */
Alexey Kardashevskiyb0e59b82013-06-21 09:43:21 -0600175 container = open("/dev/vfio/vfio", O_RDWR);
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600176
177 if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
178 /* Unknown API version */
179
Alexey Kardashevskiyb0e59b82013-06-21 09:43:21 -0600180 if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_TYPE1_IOMMU))
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600181 /* 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 */
Zi Shen Limdac09b52013-09-05 16:36:21 -0600196 ioctl(container, VFIO_SET_IOMMU, VFIO_TYPE1_IOMMU);
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600197
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
Zi Shen Limdac09b52013-09-05 16:36:21 -0600232 ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &irq);
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600233
234 /* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
235 }
236
237 /* Gratuitous device reset and go... */
238 ioctl(device, VFIO_DEVICE_RESET);
239
240VFIO User API
241-------------------------------------------------------------------------------
242
243Please see include/linux/vfio.h for complete API documentation.
244
245VFIO bus driver API
246-------------------------------------------------------------------------------
247
248VFIO bus drivers, such as vfio-pci make use of only a few interfaces
249into VFIO core. When devices are bound and unbound to the driver,
250the driver should call vfio_add_group_dev() and vfio_del_group_dev()
251respectively:
252
253extern 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
258extern void *vfio_del_group_dev(struct device *dev);
259
260vfio_add_group_dev() indicates to the core to begin tracking the
261specified iommu_group and register the specified dev as owned by
262a VFIO bus driver. The driver provides an ops structure for callbacks
263similar to a file operations structure:
264
265struct 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
277Each function is passed the device_data that was originally registered
278in the vfio_add_group_dev() call above. This allows the bus driver
279an easy place to store its opaque, private data. The open/release
280callbacks are issued when a new file descriptor is created for a
281device (via VFIO_GROUP_GET_DEVICE_FD). The ioctl interface provides
282a direct pass through for VFIO_DEVICE_* ioctls. The read/write/mmap
283interfaces implement the device region access defined by the device's
284own VFIO_DEVICE_GET_REGION_INFO ioctl.
285
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000286
287PPC64 sPAPR implementation note
288-------------------------------------------------------------------------------
289
290This implementation has some specifics:
291
2921) Only one IOMMU group per container is supported as an IOMMU group
293represents the minimal entity which isolation can be guaranteed for and
294groups are allocated statically, one per a Partitionable Endpoint (PE)
295(PE is often a PCI domain but not always).
296
2972) The hardware supports so called DMA windows - the PCI address range
298within which DMA transfer is allowed, any attempt to access address space
299out of the window leads to the whole PE isolation.
300
3013) PPC64 guests are paravirtualized but not fully emulated. There is an API
302to map/unmap pages for DMA, and it normally maps 1..32 pages per call and
303currently there is no way to reduce the number of calls. In order to make things
304faster, the map/unmap handling has been implemented in real mode which provides
305an excellent performance which has limitations such as inability to do
306locked pages accounting in real time.
307
Gavin Shan1b69be52014-06-10 11:41:57 +10003084) According to sPAPR specification, A Partitionable Endpoint (PE) is an I/O
309subtree that can be treated as a unit for the purposes of partitioning and
310error recovery. A PE may be a single or multi-function IOA (IO Adapter), a
311function of a multi-function IOA, or multiple IOAs (possibly including switch
312and bridge structures above the multiple IOAs). PPC64 guests detect PCI errors
313and recover from them via EEH RTAS services, which works on the basis of
314additional ioctl commands.
315
316So 4 additional ioctls have been added:
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000317
318 VFIO_IOMMU_SPAPR_TCE_GET_INFO - returns the size and the start
319 of the DMA window on the PCI bus.
320
321 VFIO_IOMMU_ENABLE - enables the container. The locked pages accounting
322 is done at this point. This lets user first to know what
323 the DMA window is and adjust rlimit before doing any real job.
324
325 VFIO_IOMMU_DISABLE - disables the container.
326
Gavin Shan1b69be52014-06-10 11:41:57 +1000327 VFIO_EEH_PE_OP - provides an API for EEH setup, error detection and recovery.
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000328
329The code flow from the example above should be slightly changed:
330
Gavin Shan1b69be52014-06-10 11:41:57 +1000331 struct vfio_eeh_pe_op pe_op = { .argsz = sizeof(pe_op), .flags = 0 };
332
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000333 .....
334 /* Add the group to the container */
335 ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
336
337 /* Enable the IOMMU model we want */
338 ioctl(container, VFIO_SET_IOMMU, VFIO_SPAPR_TCE_IOMMU)
339
340 /* Get addition sPAPR IOMMU info */
341 vfio_iommu_spapr_tce_info spapr_iommu_info;
342 ioctl(container, VFIO_IOMMU_SPAPR_TCE_GET_INFO, &spapr_iommu_info);
343
344 if (ioctl(container, VFIO_IOMMU_ENABLE))
345 /* Cannot enable container, may be low rlimit */
346
347 /* Allocate some space and setup a DMA mapping */
348 dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
349 MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
350
351 dma_map.size = 1024 * 1024;
352 dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
353 dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
354
355 /* Check here is .iova/.size are within DMA window from spapr_iommu_info */
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000356 ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
Gavin Shan1b69be52014-06-10 11:41:57 +1000357
358 /* Get a file descriptor for the device */
359 device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
360
361 ....
362
363 /* Gratuitous device reset and go... */
364 ioctl(device, VFIO_DEVICE_RESET);
365
366 /* Make sure EEH is supported */
367 ioctl(container, VFIO_CHECK_EXTENSION, VFIO_EEH);
368
369 /* Enable the EEH functionality on the device */
370 pe_op.op = VFIO_EEH_PE_ENABLE;
371 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
372
373 /* You're suggested to create additional data struct to represent
374 * PE, and put child devices belonging to same IOMMU group to the
375 * PE instance for later reference.
376 */
377
378 /* Check the PE's state and make sure it's in functional state */
379 pe_op.op = VFIO_EEH_PE_GET_STATE;
380 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
381
382 /* Save device state using pci_save_state().
383 * EEH should be enabled on the specified device.
384 */
385
386 ....
387
388 /* When 0xFF's returned from reading PCI config space or IO BARs
389 * of the PCI device. Check the PE's state to see if that has been
390 * frozen.
391 */
392 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
393
394 /* Waiting for pending PCI transactions to be completed and don't
395 * produce any more PCI traffic from/to the affected PE until
396 * recovery is finished.
397 */
398
399 /* Enable IO for the affected PE and collect logs. Usually, the
400 * standard part of PCI config space, AER registers are dumped
401 * as logs for further analysis.
402 */
403 pe_op.op = VFIO_EEH_PE_UNFREEZE_IO;
404 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
405
406 /*
407 * Issue PE reset: hot or fundamental reset. Usually, hot reset
408 * is enough. However, the firmware of some PCI adapters would
409 * require fundamental reset.
410 */
411 pe_op.op = VFIO_EEH_PE_RESET_HOT;
412 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
413 pe_op.op = VFIO_EEH_PE_RESET_DEACTIVATE;
414 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
415
416 /* Configure the PCI bridges for the affected PE */
417 pe_op.op = VFIO_EEH_PE_CONFIGURE;
418 ioctl(container, VFIO_EEH_PE_OP, &pe_op);
419
420 /* Restored state we saved at initialization time. pci_restore_state()
421 * is good enough as an example.
422 */
423
424 /* Hopefully, error is recovered successfully. Now, you can resume to
425 * start PCI traffic to/from the affected PE.
426 */
427
428 ....
Alexey Kardashevskiy5ffd2292013-05-21 13:33:10 +1000429
Alex Williamson4a5b2a22012-07-31 08:16:23 -0600430-------------------------------------------------------------------------------
431
432[1] VFIO was originally an acronym for "Virtual Function I/O" in its
433initial implementation by Tom Lyon while as Cisco. We've since
434outgrown the acronym, but it's catchy.
435
436[2] "safe" also depends upon a device being "well behaved". It's
437possible for multi-function devices to have backdoors between
438functions and even for single function devices to have alternative
439access to things like PCI config space through MMIO registers. To
440guard against the former we can include additional precautions in the
441IOMMU driver to group multi-function PCI devices together
442(iommu=group_mf). The latter we can't prevent, but the IOMMU should
443still provide isolation. For PCI, SR-IOV Virtual Functions are the
444best indicator of "well behaved", as these are designed for
445virtualization usage models.
446
447[3] As always there are trade-offs to virtual machine device
448assignment that are beyond the scope of VFIO. It's expected that
449future IOMMU technologies will reduce some, but maybe not all, of
450these trade-offs.
451
452[4] In this case the device is below a PCI bridge, so transactions
453from either function of the device are indistinguishable to the iommu:
454
455-[0000:00]-+-1e.0-[06]--+-0d.0
456 \-0d.1
457
45800:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)