Re: [PATCH v18 12/14] 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 "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 (including migrating opened FDs

Scratch the comma after "Note", ...

> through migration channel, which must be UNIX socket), is

... and the one before "is".

> enabled by global "local" migration parameters. But individual
> devices may have additional options to enable/disable it
> per device.
>
> The tricky thing, is that we need to know, should we do

Do you mean "The tricky thing is that we ween to know whether to call"?

> call open/connect in TAP initialization code. That means,
> that on incoming migration we want to know status of
> "local" migration parameter at time of creating TAP device.
> So, currently, for incoming local migration only TAP device
> created through QMP is supported, as you need to set migration
> parameter first (through QMP as well).
>
> So the full picture is:
>
> On source, to start outgoing "local" migration you need:
>
>  - migration parameter "local" set to true
>  - "local-migration-supported" TAP option set to true (the
>    default, starting from 11.2 QEMU Machine Types)
>
> 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 "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.

Out of curiosity, how would such a failure look like?

> Moreover, there are interface restrictions: if you create TAP
> device when QEMU is in INCOMING state, and both "local"
> and "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>
> ---
>
> In v17 thread we considered making new qapi parameters "unstable".
> Still, it was because of doubtful "incoming-fds" parameter. In this
> v18 it's completely removed, and we may live only with "local"
> migration parameter + "local-migration-supported" TAP option.
> This way the series doesn't conflict with suggested by Peter
>   [PATCH RFC 0/2] migration/vl: new -incoming config:* for early migration parameters
> So, I think, worth making one more iteration as "stable candidate".
> If still some doubts, not problem for me to resend with "unstable" features.

Up to the migration maintainers, I think.

[...]

> diff --git a/qapi/net.json b/qapi/net.json
> index ada0329ef9d..2070979f20e 100644
> --- a/qapi/net.json
> +++ b/qapi/net.json
> @@ -435,6 +435,26 @@
>  # @poll-us: maximum number of microseconds that could be spent on busy
>  #     polling for tap (since 2.7)
>  #
> +# @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 TAP backend,

Suggest "of a TAP backend", or maybe "of any 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 @local-migration-supporeted
> +#     parameter set) will simply fail.
> +#     Moreover, when QEMU is in incoming migration state, migration
> +#     parameter @local is set and @local-migration-supported is set,
> +#     following options are not supported and must not be set:

the following options

> +#     @ds, @fds, @helper, @br, @ifname, @sndbuf, @vnet_hdr.

@ds does not exist.  Do you mean @fd?

> +#     Additionally in this case @script and @downscipt must be
> +#     explicitly disabled (empty strings or "no").

"no" is deprecated.  Scratch the parenthesis?

> +#     (Since 11.2.  Defaults to true for QEMU Machine Types >= 11.2,
> +#     and to false for QEMU Machine Types < 11.2)

We commonly write this like

   #     Defaults to true for QEMU Machine Types >= 11.2, and to false
   #     for QEMU Machine Types < 11.2.  (Since 11.2)

> +#
>  # Since: 1.2
>  ##
>  { 'struct': 'NetdevTapOptions',
> @@ -453,7 +473,8 @@
>      '*vhostfds':   'str',
>      '*vhostforce': 'bool',
>      '*queues':     'uint32',
> -    '*poll-us':    'uint32'} }
> +    '*poll-us':    'uint32',
> +    '*local-migration-supported': 'bool' } }
>  
>  ##
>  # @NetdevSocketOptions: