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

[Erik Skultety]

On Mon, Apr 13, 2020 at 01:55:04AM -0400, Yan Zhao wrote:
> 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@xxxxxxxxxx>
> Cc: Erik Skultety <eskultet@xxxxxxxxxx>
> Cc: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx>
> Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
> Cc: Zhenyu Wang <zhenyuw@xxxxxxxxxxxxxxx>
> Cc: "Wang, Zhi A" <zhi.a.wang@xxxxxxxxx>
> Cc: Neo Jia <cjia@xxxxxxxxxx>
> Cc: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> Cc: Daniel P. Berrangà <berrange@xxxxxxxxxx>
> Cc: Christophe de Dinechin <dinechin@xxxxxxxxxx>
>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> ---
>  .../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)

Hmm, ^this is not consistent with how patch 1/5 reports this information, but
looking at the existing docs we're not doing very well in terms of consistency
there either.

I suggest we go with "(read-write)" in both patch 1/5 and here and then start
the paragraph with "This is an optional attribute."

> +  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

after the target device has been created.

side note: maybe add something like "(see the migration_version attribute of
the device node if the target device already exists)" in the same section in
patch 1/5.

> +  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;

s/devicedoes/device does/

> +  -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
>  ------------------------

Overall, the same comments as in 1/5 apply text-wise.

Regards,
--
Erik Skultety