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Wed, 29 Jul 2026 11:30:58 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.4]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 20D25414; Wed, 29 Jul 2026 11:30:57 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 9F16121E6920; Wed, 29 Jul 2026 13:30:54 +0200 (CEST) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Cc: jasowang@redhat.com, mst@redhat.com, armbru@redhat.com, peterx@redhat.com, farosas@suse.de, raphael.s.norwitz@gmail.com, bchaney@akamai.com, qemu-devel@nongnu.org, berrange@redhat.com, pbonzini@redhat.com, yc-core@yandex-team.ru, mark.caveayland@nutanix.com, Jason Wang , Eric Blake Subject: Re: [PATCH v20 13/15] net/tap: support local migration with virtio-net In-Reply-To: <20260729091334.1863155-14-vsementsov@yandex-team.ru> (Vladimir Sementsov-Ogievskiy's message of "Wed, 29 Jul 2026 12:12:51 +0300") References: <20260729091334.1863155-1-vsementsov@yandex-team.ru> <20260729091334.1863155-14-vsementsov@yandex-team.ru> Date: Wed, 29 Jul 2026 13:30:54 +0200 Message-ID: <87jyqeqbgx.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -2 X-Spam_score: -0.3 X-Spam_bar: / X-Spam_report: (-0.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.58, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Vladimir Sementsov-Ogievskiy 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 > Reviewed-by: Ben Chaney [...] > 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. ...