Re: [PATCH v21 15/16] net/tap: support local migration with virtio-net

[Markus Armbruster <armbru@redhat.com>]

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

> Support transferring of TAP state (including open fd).
>
> Add new property "x-permit-local-migration", which defines whether
> local migration is actually permitted for this TAP device.
>
> Note that local-migration is enabled by global "local" migration
> parameter, but individual devices may have additional options to
> enable/disable it per device.
>
> The tricky thing is that we need to know whether to call open/connect in
> TAP initialization code, i.e. we need to know the value of migration
> parameter "local" when creating the TAP device.  For incoming migration,
> we can know only for TAP devices created with QMP after setting the
> migration parameter with QMP.
>
> So the full picture is:
>
> On source, to start outgoing "local" migration you need:
>
>  - migration parameter "local" set to true
>  - "x-permit-local-migration" TAP option set to true
>
> If at least one of these options is not set, TAP backend
> doesn't participate in migration.
>
> On target, things are more difficult:
>
> Same, you need both "local" and "x-permit-local-migration"
> be set. And same, if one of them is not set, TAP backend
> is initialized as usual, and doesn't accept any incoming
> state.
>
> Additionally, if you are going to set "local", it must be
> set before creating the TAP device. If TAP device created
> with "local" unset, it initializes as usual. If you enable
> "local" after it and start incoming migration, it will fail
> in .pre_load handler of TAP backend.
>
> Moreover, there are interface restrictions: if you create TAP
> device when QEMU is in INCOMING state, and both "local"
> and "x-permit-local-migration" set, most of TAP options are
> not allowed, and script/downscript are required to be explicitly
> disabled.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Reviewed-by: Ben Chaney <bchaney@akamai.com>

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>