Re: [PATCH v8 16/19] qapi: introduce backend-transfer migration parameter

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> This parameter enables backend-transfer feature: all devices
> which support it will migrate their backends (for example a TAP
> device, by passing open file descriptor to migration channel).
>
> Currently no such devices, so the new parameter is a noop.
>
> Next commit will add support for virtio-net, to migrate its
> TAP backend.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

[...]

> diff --git a/qapi/migration.json b/qapi/migration.json
> index be0f3fcc12..35601a1f87 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -951,9 +951,16 @@
>  #     is @cpr-exec.  The first list element is the program's filename,
>  #     the remainder its arguments.  (Since 10.2)
>  #
> +# @backend-transfer: Enable backend-transfer feature for devices that

Either "Enable the backend transfer feature" or "Enable backend transfer"

> +#     supports it. In general that means that backend state and its

support

> +#     file descriptors are passed to the destination in the migraton
> +#     channel (which must be a UNIX socket). Individual devices
> +#     declare the support for backend-transfer by per-device
> +#     backend-transfer option. (Since 10.2)
> +#

I'm not sure I understand this.

What is a "per-device backend-transfer option"?  Is it a device
property?

If yes, I guess the device declares its capability to do this by having
this property.  Correct?

Does the property's value matter?  How?

>  # Features:
>  #
> -# @unstable: Members @x-checkpoint-delay and
> +# @unstable: Members @backend-transfer, @x-checkpoint-delay and
>  #     @x-vcpu-dirty-limit-period are experimental.
>  #
>  # Since: 2.4
> @@ -978,7 +985,8 @@
>             'mode',
>             'zero-page-detection',
>             'direct-io',
> -           'cpr-exec-command'] }
> +           'cpr-exec-command',
> +           { 'name': 'backend-transfer', 'features': ['unstable'] } ] }
>  
>  ##
>  # @MigrateSetParameters:

[...]