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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4950a2e98b0sm107671795e9.8.2026.07.14.01.03.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Jul 2026 01:03:06 -0700 (PDT) Date: Tue, 14 Jul 2026 04:03:03 -0400 From: "Michael S. Tsirkin" To: Vladimir Sementsov-Ogievskiy Cc: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , jasowang@redhat.com, armbru@redhat.com, peterx@redhat.com, farosas@suse.de, raphael.s.norwitz@gmail.com, bchaney@akamai.com, qemu-devel@nongnu.org, pbonzini@redhat.com, yc-core@yandex-team.ru, mark.caveayland@nutanix.com, Eric Blake Subject: Re: [PATCH v18 09/14] qapi: add local migration parameter Message-ID: <20260714040236-mutt-send-email-mst@kernel.org> References: <20260710191809.721154-1-vsementsov@yandex-team.ru> <20260710191809.721154-10-vsementsov@yandex-team.ru> <05c89310-7bec-4aac-addc-b5ef377e09a7@yandex-team.ru> <25b5e9a6-e563-4ab0-9075-04a3a00d3b33@yandex-team.ru> <20260713162057-mutt-send-email-mst@kernel.org> <37a769c6-b70d-4452-bdbc-d37a7ebcc732@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <37a769c6-b70d-4452-bdbc-d37a7ebcc732@yandex-team.ru> Received-SPF: permerror client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_H2=-0.01, SPF_HELO_PASS=-0.001, T_SPF_PERMERROR=0.01 autolearn=ham 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 On Tue, Jul 14, 2026 at 09:50:34AM +0300, Vladimir Sementsov-Ogievskiy wrote: > On 13.07.26 23:22, Michael S. Tsirkin wrote: > > On Mon, Jul 13, 2026 at 07:37:50PM +0100, Daniel P. Berrangé wrote: > > > On Mon, Jul 13, 2026 at 08:55:37PM +0300, Vladimir Sementsov-Ogievskiy wrote: > > > > On 13.07.26 14:21, Daniel P. Berrangé wrote: > > > > > On Mon, Jul 13, 2026 at 01:55:48PM +0300, Vladimir Sementsov-Ogievskiy wrote: > > > > > > On 13.07.26 12:51, Daniel P. Berrangé wrote: > > > > > > > On Fri, Jul 10, 2026 at 10:17:58PM +0300, Vladimir Sementsov-Ogievskiy wrote: > > > > > > > > We are going to implement local-migration feature: some devices will be > > > > > > > > able to transfer open file descriptors through migration stream (which > > > > > > > > must UNIX domain socket for that purpose). This allows to transfer the > > > > > > > > whole backend state without reconnecting and restarting the backend > > > > > > > > service. For example, virtio-net will migrate its attached TAP netdev, > > > > > > > > together with its connected file descriptors. > > > > > > > > > > > > > > > > In this commit we introduce a migration parameter, which enables > > > > > > > > the feature for devices that support it (none at the moment). > > > > > > > > > > > > > > > > Signed-off-by: Vladimir Sementsov-Ogievskiy > > > > > > > > Acked-by: Markus Armbruster > > > > > > > > Acked-by: Peter Xu > > > > > > > > Reviewed-by: Ben Chaney > > > > > > > > --- > > > > > > > > include/migration/misc.h | 2 ++ > > > > > > > > migration/options.c | 18 +++++++++++++++++- > > > > > > > > qapi/migration.json | 12 ++++++++++-- > > > > > > > > 3 files changed, 29 insertions(+), 3 deletions(-) > > > > > > > > > > > > > > > > > > > > > > diff --git a/qapi/migration.json b/qapi/migration.json > > > > > > > > index 66bd900465a..03bdf19f3a6 100644 > > > > > > > > --- a/qapi/migration.json > > > > > > > > +++ b/qapi/migration.json > > > > > > > > @@ -830,7 +830,8 @@ > > > > > > > > 'zero-page-detection', > > > > > > > > 'direct-io', > > > > > > > > { 'name': 'x-rdma-chunk-size', 'features': [ 'unstable' ] }, > > > > > > > > - 'cpr-exec-command'] } > > > > > > > > + 'cpr-exec-command', > > > > > > > > + 'local'] } > > > > > > > > ## > > > > > > > > # @migrate-set-parameters: > > > > > > > > @@ -1012,6 +1013,12 @@ > > > > > > > > # Must be set to the same value on both source and destination > > > > > > > > # before migration starts. (Since 11.1) > > > > > > > > # > > > > > > > > +# @local: Enable local migration for devices that support it. Backend > > > > > > > > +# state and its file descriptors can then be passed to the > > > > > > > > +# destination in the migration channel. The migration channel > > > > > > > > +# must be a Unix domain socket. Usually needs to be enabled per > > > > > > > > +# device. (Since 11.2) > > > > > > > > > > > > > > What happens when this is enabled and there are devices which > > > > > > > don't support it ? > > > > > > > > > > > > These devices are migrated as usual, ignoring "local" option. > > > > > > > > > > > > > > > > > > > > What happens when this is enabled and a device on the src > > > > > > > does not support it but a device on the (newer) QEMU dest > > > > > > > does support it ? How does the mgmt app know which devices > > > > > > > support it ? > > > > > > > > > > > > The only supporting device (TAP-backend) declares support by > > > > > > "local-migration-supported", so mgmt app should care to set > > > > > > same value for it on host and target (like for other options > > > > > > actually). Or simply relay on default, which is true starting > > > > > > from 11.2 Machine Type. > > > > > > > > > > > > > > > > > > > > Can we document the semantics in these cases. > > > > > > > > > > > > That's: "Usually needs to be enabled per device." > > > > > > > > > > ....except that we have it enabled by default for TAP backends > > > > > in new machine types.... except that it will then break if the > > > > > app/user has enabled any of the "forbidden" options. > > > > > > > > > > The way this doc is written makes it sound like "local" is an > > > > > optimization you can turn on in any scenario, and it will do > > > > > the right thing if the device supports it. That is very much > > > > > not the case - enabling this flag will definitely break many > > > > > configs out of the box, with new machine types due to the TAP > > > > > device defaults & incompatible options. > > > > > > > > Ok, now we decided to NOT make default support for TAP in newer > > > > machine types. > > > > > > > > So, enabling "local" is safe in existing scenarios, only explicit > > > > enabling TAP.local-migration-supported will change the behavior. > > > > > > > > Keep "local" documentation as is? > > > > > > > > I see the only way to make it more informative: directly describe > > > > connection with TAP.local-migration-supported, without general > > > > phrases about "devices that support" and "usual". We'll have to > > > > update it when more devices added (my next step is vhost-user-blk), > > > > but at least, it would be absolutely correct at the moment. > > > > > > How about.... > > > > > > # @local: Permit the use of optimizations for local migration. > > > # This must only be set when both the source and destination > > > # QEMU processes are on the same OS and directly connected > > > # with a UNIX domain socket as the migration channel to enable > > > # use of file descriptor passing. Individual device backends > > > # may need additional configuration flags set to enable local > > > # migration optimizations. This will be documented against the > > > # device backends where it applies. (Since 11.2) > > > > > > > BTW surely qemu can just detect this? getsockname maybe? > > > > Yes, but this doesn't guarantee that it's a direct connection by UNIX > socket to target, and it really can pass FDs. UNIX socket may be only > a first part of transfer channel to the remote target. Sure, but that's not a reason not to check so we can fail early. > -- > Best regards, > Vladimir