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vfio.txt: standardize document format
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Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:
- adjust title marks;
- use footnote marks;
- mark literal blocks;
- adjust identation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mauro Carvalho Chehab authored and Jonathan Corbet committed Jul 14, 2017
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281 changes: 144 additions & 137 deletions Documentation/vfio.txt
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VFIO - "Virtual Function I/O"[1]
-------------------------------------------------------------------------------
==================================
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.
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].
bare-metal device drivers [3]_.

Some applications, particularly in the high performance computing
field, also benefit from low-overhead, direct device access from
Expand All @@ -31,7 +33,7 @@ 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,
Expand Down Expand Up @@ -114,40 +116,40 @@ well as mechanisms for describing and registering interrupt
notifications.

VFIO Usage Example
-------------------------------------------------------------------------------
------------------

Assume user wants to access PCI device 0000:06:0d.0
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
$ 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:
group::

# modprobe vfio-pci
# modprobe vfio-pci

Binding this device to the vfio-pci driver creates the VFIO group
character devices for this 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
$ 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
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
Expand All @@ -157,12 +159,12 @@ 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).
mode 0666 by the system)::

# chown user:user /dev/vfio/26
# 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:
group and can access them as follows::

int container, group, device, i;
struct vfio_group_status group_status =
Expand Down Expand Up @@ -248,31 +250,31 @@ 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:
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 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);
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);
};
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
Expand All @@ -285,50 +287,55 @@ 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.
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.
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.
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.
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:
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_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
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_IOMMU_DISABLE
disables the container.

VFIO_EEH_PE_OP - provides an API for EEH setup, error detection and recovery.
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:
The code flow from the example above should be slightly changed::

struct vfio_eeh_pe_op pe_op = { .argsz = sizeof(pe_op), .flags = 0 };

Expand Down Expand Up @@ -442,73 +449,73 @@ The code flow from the example above should be slightly changed:
....

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).
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:
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_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.
- 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.
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.
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)
.. [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)

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