[PATCH v5 3/4] vfio/mdev: add migration_version attribute for mdev (under mdev device node)

[Yan Zhao]

migration_version attribute is used to check migration compatibility
between two mdev devices of the same mdev type.
The key is that it's rw and its data is opaque to userspace.

Userspace reads migration_version of mdev device at source side and
writes the value to migration_version attribute of mdev device at target
side. It judges migration compatibility according to whether the read
and write operations succeed or fail.

Currently, it is able to read/write migration_version attribute under two
places:

(1) under mdev_type node
userspace is able to know whether two mdev devices are compatible before
a mdev device is created.

userspace also needs to check whether the two mdev devices are of the same
mdev type before checking the migration_version attribute. It also needs
to check device creation parameters if aggregation is supported in future.

(2) under mdev device node
userspace is able to know whether two mdev devices are compatible after
they are all created. But it does not need to check mdev type and device
creation parameter for aggregation as device vendor driver would have
incorporated those information into the migration_version attribute.

             __    userspace
              /\              \
             /                 \write
            / read              \
   ________/__________       ___\|/_____________
  | migration_version |     | migration_version |-->check migration
  ---------------------     ---------------------   compatibility
    mdev device A               mdev device B

This patch is for mdev documentation about the second place (under
mdev device node)

Cc: Alex Williamson <alex.williamson at redhat.com>
Cc: Erik Skultety <eskultet at redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert at redhat.com>
Cc: Cornelia Huck <cohuck at redhat.com>
Cc: "Tian, Kevin" <kevin.tian at intel.com>
Cc: Zhenyu Wang <zhenyuw at linux.intel.com>
Cc: "Wang, Zhi A" <zhi.a.wang at intel.com>
Cc: Neo Jia <cjia at nvidia.com>
Cc: Kirti Wankhede <kwankhede at nvidia.com>
Cc: Daniel P. Berrangé <berrange at redhat.com>
Cc: Christophe de Dinechin <dinechin at redhat.com>

Signed-off-by: Yan Zhao <yan.y.zhao at intel.com>
---
 .../driver-api/vfio-mediated-device.rst       | 70 +++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/Documentation/driver-api/vfio-mediated-device.rst b/Documentation/driver-api/vfio-mediated-device.rst
index 2d1f3c0f3c8f..efbadfd51b7e 100644
--- a/Documentation/driver-api/vfio-mediated-device.rst
+++ b/Documentation/driver-api/vfio-mediated-device.rst
@@ -383,6 +383,7 @@ Directories and Files Under the sysfs for Each mdev Device
          |--- remove
          |--- mdev_type {link to its type}
          |--- vendor-specific-attributes [optional]
+         |--- migration_verion [optional]
 
 * remove (write only)
 
@@ -394,6 +395,75 @@ Example::
 
 	# echo 1 > /sys/bus/mdev/devices/$mdev_UUID/remove
 
+* migration_version (rw, optional)
+  It is used to check migration compatibility between two mdev devices.
+  Absence of this attribute means the mdev device does not support migration.
+
+  This attribute provides a way to check migration compatibility between two
+  mdev devices from userspace after device created. The intended usage is
+  for userspace to read the migration_version attribute from one mdev device and
+  then writing that value to the migration_version attribute of the other mdev
+  device. The second mdev device indicates compatibility via the return code of
+  the write operation. This makes compatibility between mdev devices completely
+  vendor-defined and opaque to userspace. Userspace should do nothing more
+  than use the migration_version attribute to confirm source to target
+  compatibility.
+
+  Reading/Writing Attribute Data:
+  read(2) will fail if a mdev device does not support migration and otherwise
+        succeed and return migration_version string of the mdev device.
+
+        This migration_version string is vendor defined and opaque to the
+        userspace. Vendor is free to include whatever they feel is relevant.
+        e.g. <pciid of parent device>-<software version>.
+
+        Restrictions on this migration_version string:
+            1. It should only contain ascii characters
+            2. MAX Length is PATH_MAX (4096)
+
+  write(2) expects migration_version string of source mdev device, and will
+         succeed if it is determined to be compatible and otherwise fail with
+         vendor specific errno.
+
+  Errno:
+  -An errno on read(2) indicates the mdev devicedoes not support migration;
+  -An errno on write(2) indicates the mdev devices are incompatible or the
+   target doesn't support migration.
+  Vendor driver is free to define specific errno and is suggested to
+  print detailed error in syslog for diagnose purpose.
+
+  Userspace should treat ANY of below conditions as two mdev devices not
+  compatible:
+  (1) any one of the two mdev devices does not have a migration_version
+  attribute
+  (2) error when reading from migration_version attribute of one mdev device
+  (3) error when writing migration_version string of one mdev device to
+  migration_version attribute of the other mdev device
+
+  Userspace should regard two mdev devices compatible when ALL of below
+  conditions are met:
+  (1) success when reading from migration_version attribute of one mdev device.
+  (2) success when writing migration_version string of one mdev device to
+  migration_version attribute of the other mdev device.
+
+  Example Usage:
+  (1) Retrieve the mdev source migration_version:
+
+  # cat /sys/bus/mdev/devices/$mdev_UUID1/migration_version
+
+  If reading the source migration_version generates an error, migration is not
+  possible.
+
+  (2) Test source migration_version at target:
+
+  Given a migration_version as outlined above, its compatibility to an
+  instantiated device of the same mdev type can be tested as:
+  # echo $VERSION > /sys/bus/mdev/devices/$mdev_UUID2/migration_version
+
+  If this write fails, the source and target migration versions are not
+  compatible or the target does not support migration.
+
+
 Mediated device Hot plug
 ------------------------
 
-- 
2.17.1