Re: [PATCH QEMU v25 17/17] qapi: Add VFIO devices migration stats in Migration stats

[Markus Armbruster <armbru@redhat.com>]

QAPI review only.

The only changes since I reviewed v23 is the rename of VfioStats member
@bytes to @transferred, and the move of MigrationInfo member @vfio next
to @ram and @disk.  Good.  I'm copying my other questions in the hope of
getting answers :)

Kirti Wankhede <kwankhede@nvidia.com> writes:

> Added amount of bytes transferred to the target VM by all VFIO devices
>
> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
[...]
> diff --git a/qapi/migration.json b/qapi/migration.json
> index d5000558c6c9..952864b05455 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -146,6 +146,18 @@
>              'active', 'postcopy-active', 'postcopy-paused',
>              'postcopy-recover', 'completed', 'failed', 'colo',
>              'pre-switchover', 'device', 'wait-unplug' ] }
> +##
> +# @VfioStats:
> +#
> +# Detailed VFIO devices migration statistics
> +#
> +# @transferred: amount of bytes transferred to the target VM by VFIO devices
> +#
> +# Since: 5.1
> +#
> +##
> +{ 'struct': 'VfioStats',
> +  'data': {'transferred': 'int' } }

Pardon my ignorance...  What exactly do VFIO devices transfer to the
target VM?  How is that related to MigrationInfo member @ram?

MigrationStats has much more information, and some of it is pretty
useful to track how migration is doing, in particular whether it
converges, and how fast.  Absent in VfioStats due to "not implemented",
or due to "can't be done"?

Byte counts should use QAPI type 'size'.  Many existing ones don't.
Since MigrationStats uses 'int', I'll let the migration maintainers
decide whether they want 'int' or 'size' here.

>  ##
>  # @MigrationInfo:
> @@ -207,11 +219,16 @@
>  #
>  # @socket-address: Only used for tcp, to know what the real port is (Since 4.0)
>  #
> +# @vfio: @VfioStats containing detailed VFIO devices migration statistics,
> +#        only returned if VFIO device is present, migration is supported by all
> +#         VFIO devices and status is 'active' or 'completed' (since 5.1)
> +#
>  # Since: 0.14.0
>  ##
>  { 'struct': 'MigrationInfo',
>    'data': {'*status': 'MigrationStatus', '*ram': 'MigrationStats',
>             '*disk': 'MigrationStats',
> +           '*vfio': 'VfioStats',
>             '*xbzrle-cache': 'XBZRLECacheStats',
>             '*total-time': 'int',
>             '*expected-downtime': 'int',