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Mon, 13 Jul 2026 07:58:01 +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 C0DAE36F33; Mon, 13 Jul 2026 07:57:59 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 5494A21E6920; Mon, 13 Jul 2026 09:57:57 +0200 (CEST) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Cc: jasowang@redhat.com, mst@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, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Zhao Liu , Jason Wang , Eric Blake Subject: Re: [PATCH v18 12/14] net/tap: support local migration with virtio-net In-Reply-To: <20260710191809.721154-13-vsementsov@yandex-team.ru> (Vladimir Sementsov-Ogievskiy's message of "Fri, 10 Jul 2026 22:18:01 +0300") References: <20260710191809.721154-1-vsementsov@yandex-team.ru> <20260710191809.721154-13-vsementsov@yandex-team.ru> Date: Mon, 13 Jul 2026 09:57:57 +0200 Message-ID: <875x2juxru.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: permerror client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 12 X-Spam_score: 1.2 X-Spam_bar: + X-Spam_report: (1.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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=-0.01, RCVD_IN_MSPIKE_WL=-0.01, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, T_SPF_PERMERROR=0.01 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 "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 > --- > > 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: