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authorMauro Carvalho Chehab <mchehab+samsung@kernel.org>2019-06-27 20:39:22 +0200
committerMauro Carvalho Chehab <mchehab+samsung@kernel.org>2019-07-15 16:03:02 +0200
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tree29e0400c806016783a3fd7a380be40a201956653 /Documentation/vfio.txt
parentdocs: admin-guide: add a series of orphaned documents (diff)
downloadlinux-baa293e9544bea71361950d071579f0e4d5713ed.tar.xz
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docs: driver-api: add a series of orphaned documents
There are lots of documents under Documentation/*.txt and a few other orphan documents elsehwere that belong to the driver-API book. Move them to their right place. Reviewed-by: Cornelia Huck <cohuck@redhat.com> # vfio-related parts Acked-by: Logan Gunthorpe <logang@deltatee.com> # switchtec Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
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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
-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_type1_info iommu_info = { .argsz = sizeof(iommu_info) };
- struct vfio_iommu_type1_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, &irq);
-
- /* 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 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
-iommu_group of the specified dev and register the 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.
-
-
-PPC64 sPAPR implementation note
--------------------------------
-
-This implementation has some specifics:
-
-1) On older systems (POWER7 with P5IOC2/IODA1) only one IOMMU group per
- container is supported as an IOMMU table is allocated at the boot time,
- one table per a IOMMU group which is a Partitionable Endpoint (PE)
- (PE is often a PCI domain but not always).
-
- Newer systems (POWER8 with IODA2) have improved hardware design which allows
- to remove this limitation and have multiple IOMMU groups per a VFIO
- container.
-
-2) The hardware supports so called DMA windows - the PCI address range
- within which DMA transfer is allowed, any attempt to access address space
- out of the window leads to the whole PE isolation.
-
-3) PPC64 guests are paravirtualized but not fully emulated. There is an API
- to map/unmap pages for DMA, and it normally maps 1..32 pages per call and
- currently there is no way to reduce the number of calls. In order to make
- things faster, the map/unmap handling has been implemented in real mode
- which provides an excellent performance which has limitations such as
- inability to do locked pages accounting in real time.
-
-4) According to sPAPR specification, A Partitionable Endpoint (PE) is an I/O
- subtree that can be treated as a unit for the purposes of partitioning and
- error recovery. A PE may be a single or multi-function IOA (IO Adapter), a
- function of a multi-function IOA, or multiple IOAs (possibly including
- switch and bridge structures above the multiple IOAs). PPC64 guests detect
- PCI errors and recover from them via EEH RTAS services, which works on the
- basis of additional ioctl commands.
-
- So 4 additional ioctls have been added:
-
- VFIO_IOMMU_SPAPR_TCE_GET_INFO
- returns the size and the start of the DMA window on the PCI bus.
-
- VFIO_IOMMU_ENABLE
- enables the container. The locked pages accounting
- is done at this point. This lets user first to know what
- the DMA window is and adjust rlimit before doing any real job.
-
- VFIO_IOMMU_DISABLE
- disables the container.
-
- VFIO_EEH_PE_OP
- provides an API for EEH setup, error detection and recovery.
-
- The code flow from the example above should be slightly changed::
-
- struct vfio_eeh_pe_op pe_op = { .argsz = sizeof(pe_op), .flags = 0 };
-
- .....
- /* 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_SPAPR_TCE_IOMMU)
-
- /* Get addition sPAPR IOMMU info */
- vfio_iommu_spapr_tce_info spapr_iommu_info;
- ioctl(container, VFIO_IOMMU_SPAPR_TCE_GET_INFO, &spapr_iommu_info);
-
- if (ioctl(container, VFIO_IOMMU_ENABLE))
- /* Cannot enable container, may be low rlimit */
-
- /* 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;
-
- /* Check here is .iova/.size are within DMA window from spapr_iommu_info */
- 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");
-
- ....
-
- /* Gratuitous device reset and go... */
- ioctl(device, VFIO_DEVICE_RESET);
-
- /* Make sure EEH is supported */
- ioctl(container, VFIO_CHECK_EXTENSION, VFIO_EEH);
-
- /* Enable the EEH functionality on the device */
- pe_op.op = VFIO_EEH_PE_ENABLE;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /* You're suggested to create additional data struct to represent
- * PE, and put child devices belonging to same IOMMU group to the
- * PE instance for later reference.
- */
-
- /* Check the PE's state and make sure it's in functional state */
- pe_op.op = VFIO_EEH_PE_GET_STATE;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /* Save device state using pci_save_state().
- * EEH should be enabled on the specified device.
- */
-
- ....
-
- /* Inject EEH error, which is expected to be caused by 32-bits
- * config load.
- */
- pe_op.op = VFIO_EEH_PE_INJECT_ERR;
- pe_op.err.type = EEH_ERR_TYPE_32;
- pe_op.err.func = EEH_ERR_FUNC_LD_CFG_ADDR;
- pe_op.err.addr = 0ul;
- pe_op.err.mask = 0ul;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- ....
-
- /* When 0xFF's returned from reading PCI config space or IO BARs
- * of the PCI device. Check the PE's state to see if that has been
- * frozen.
- */
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /* Waiting for pending PCI transactions to be completed and don't
- * produce any more PCI traffic from/to the affected PE until
- * recovery is finished.
- */
-
- /* Enable IO for the affected PE and collect logs. Usually, the
- * standard part of PCI config space, AER registers are dumped
- * as logs for further analysis.
- */
- pe_op.op = VFIO_EEH_PE_UNFREEZE_IO;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /*
- * Issue PE reset: hot or fundamental reset. Usually, hot reset
- * is enough. However, the firmware of some PCI adapters would
- * require fundamental reset.
- */
- pe_op.op = VFIO_EEH_PE_RESET_HOT;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
- pe_op.op = VFIO_EEH_PE_RESET_DEACTIVATE;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /* Configure the PCI bridges for the affected PE */
- pe_op.op = VFIO_EEH_PE_CONFIGURE;
- ioctl(container, VFIO_EEH_PE_OP, &pe_op);
-
- /* Restored state we saved at initialization time. pci_restore_state()
- * is good enough as an example.
- */
-
- /* Hopefully, error is recovered successfully. Now, you can resume to
- * start PCI traffic to/from the affected PE.
- */
-
- ....
-
-5) There is v2 of SPAPR TCE IOMMU. It deprecates VFIO_IOMMU_ENABLE/
- VFIO_IOMMU_DISABLE and implements 2 new ioctls:
- VFIO_IOMMU_SPAPR_REGISTER_MEMORY and VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY
- (which are unsupported in v1 IOMMU).
-
- PPC64 paravirtualized guests generate a lot of map/unmap requests,
- and the handling of those includes pinning/unpinning pages and updating
- mm::locked_vm counter to make sure we do not exceed the rlimit.
- The v2 IOMMU splits accounting and pinning into separate operations:
-
- - VFIO_IOMMU_SPAPR_REGISTER_MEMORY/VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY ioctls
- receive a user space address and size of the block to be pinned.
- Bisecting is not supported and VFIO_IOMMU_UNREGISTER_MEMORY is expected to
- be called with the exact address and size used for registering
- the memory block. The userspace is not expected to call these often.
- The ranges are stored in a linked list in a VFIO container.
-
- - VFIO_IOMMU_MAP_DMA/VFIO_IOMMU_UNMAP_DMA ioctls only update the actual
- IOMMU table and do not do pinning; instead these check that the userspace
- address is from pre-registered range.
-
- This separation helps in optimizing DMA for guests.
-
-6) sPAPR specification allows guests to have an additional DMA window(s) on
- a PCI bus with a variable page size. Two ioctls have been added to support
- this: VFIO_IOMMU_SPAPR_TCE_CREATE and VFIO_IOMMU_SPAPR_TCE_REMOVE.
- The platform has to support the functionality or error will be returned to
- the userspace. The existing hardware supports up to 2 DMA windows, one is
- 2GB long, uses 4K pages and called "default 32bit window"; the other can
- be as big as entire RAM, use different page size, it is optional - guests
- create those in run-time if the guest driver supports 64bit DMA.
-
- VFIO_IOMMU_SPAPR_TCE_CREATE receives a page shift, a DMA window size and
- a number of TCE table levels (if a TCE table is going to be big enough and
- the kernel may not be able to allocate enough of physically contiguous
- memory). It creates a new window in the available slot and returns the bus
- address where the new window starts. Due to hardware limitation, the user
- space cannot choose the location of DMA windows.
-
- VFIO_IOMMU_SPAPR_TCE_REMOVE receives the bus start address of the window
- and removes it.
-
--------------------------------------------------------------------------------
-
-.. [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)