Re: [PATCH v20 13/15] 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-local-migration-supported", which defines
> whether local-migration is actually supported for this TAP device.
> Starting from 11.2 QEMU Machine Types it's enabled by default.
>
> 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-local-migration-supported" TAP option set to true (the
>    default, starting from 11.2 QEMU Machine Types)

Is the machine type part still accurate?  The description in the QAPI
schema has (default: false, since 11.2).

>
> 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-local-migration-supported"
> 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-local-migration-supported" set, most of TAP options are
> not allowed, and script/downscript are required to be explicitly
> unset (set to "" or "no").
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Reviewed-by: Ben Chaney <bchaney@akamai.com>

[...]

> diff --git a/qapi/net.json b/qapi/net.json
> index acb8594c952..e6921912e12 100644
> --- a/qapi/net.json
> +++ b/qapi/net.json
> @@ -437,6 +437,29 @@
>  # @poll-us: maximum number of microseconds that could be spent on busy
>  #     polling for tap (since 2.7)
>  #
> +# @x-local-migration-supported: enable local migration for this TAP
> +#     backend.  When set, local migration is enabled/disabled by
> +#     migration parameter @local for this TAP backend.  When unset,
> +#     migration parameter @local is ignored for this TAP backend.
> +#     To be able to do incoming local migration of a TAP backend,
> +#     migration parameter @local must be set _before_ creating the
> +#     TAP backend.  Otherwise, TAP backend is initialized as usual,
> +#     opening/creating TAP devices in kernel.  In this case further
> +#     local incoming migration (with migration parameter @local set
> +#     after creating TAP backend with @x-local-migration-supporeted
> +#     parameter set) will simply fail.

Either have a a blank line here so you actually get two paragraphs, or
refill the entire description to avoid the illusion of two paragraphs.

> +#     Moreover, when QEMU is in incoming migration state, migration
> +#     parameter @local is set and @x-local-migration-supported is set,
> +#     the following options are not supported and must not be set:
> +#     @fd, @fds, @helper, @br, @ifname, @sndbuf, @vnet_hdr.
> +#     Additionally in this case @script and @downscipt must be

@downscript

> +#     explicitly disabled (empty strings or "no").

"no" is deprecated [PATCH 3].  We'll have to remember deleting 'or "no"'
here when remove it.  Easy to forget.  Delete it now?

Maybe

   #     Additionally, @script and @downscript must be explicitly disabled
   #     then.

> +#     (default: false, since 11.2)
> +#
> +# Features:
> +#
> +# @unstable: Member @x-local-migration-supported is experimental.
> +#
>  # Since: 1.2
>  ##
>  { 'struct': 'NetdevTapOptions',
> @@ -455,7 +478,9 @@
>      '*vhostfds':   'str',
>      '*vhostforce': 'bool',
>      '*queues':     'uint32',
> -    '*poll-us':    'uint32'} }
> +    '*poll-us':    'uint32',
> +    '*x-local-migration-supported': {
> +      'type': 'bool', 'features' : [ 'unstable'] } } }
>  
>  ##
>  # @NetdevSocketOptions:

Naming is hard...

"Supported" sounds like a property of the QEMU process.  That's not what
this is.  It's an on/off switch that happens to be in series with
another on/off switch, namely migration parameter @local.

Maybe

    @permit-local-migration: permit local migration for this TAP
        backend.  When set, local migration is enabled/disabled by
        migration parameter @local for this TAP backend.  ...