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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4830513730esm66577855e9.14.2026.02.03.05.29.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Feb 2026 05:29:05 -0800 (PST) From: Stefano Brivio To: Laurent Vivier Cc: Juraj Marcin , qemu-devel@nongnu.org, Fabiano Rosas , "Michael S. Tsirkin" , Peter Xu , Jason Wang , Vladimir Sementsov-Ogievskiy , Laurent Vivier , David Gibson , Cindy Lu Subject: Re: [PATCH 0/4] migration: Pass network packets received during switchover to dest VM Message-ID: <20260203142854.07640396@elisabeth> In-Reply-To: <3806403b-a937-4b10-8bfb-bc4386b7fd82@redhat.com> References: <20260127140316.4187221-1-jmarcin@redhat.com> <20260127192108.216495da@elisabeth> <3806403b-a937-4b10-8bfb-bc4386b7fd82@redhat.com> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Date: Tue, 03 Feb 2026 14:29:05 +0100 (CET) Received-SPF: pass client-ip=170.10.129.124; envelope-from=sbrivio@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=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 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, 3 Feb 2026 13:03:26 +0100 Laurent Vivier wrote: > On 1/27/26 19:21, Stefano Brivio wrote: > > [Cc'ing Laurent and David] > > > > On Tue, 27 Jan 2026 15:03:06 +0100 > > Juraj Marcin wrote: > > > >> During switchover there is a period during which both source and > >> destination side VMs are paused. During this period, all network packets > >> are still routed to the source side, but it will never process them. > >> Once the destination resumes, it is not aware of these packets and they > >> are lost. This can cause packet loss in unreliable protocols and > >> extended delays due to retransmission in reliable protocols. > >> > >> This series resolves this problem by caching packets received once the > >> source VM pauses and then passing and injecting them on the destination > >> side. This feature is implemented in the last patch. The caching and > >> injecting is implemented using network filter interface and should work > >> with any backend with vhost=off, but only TAP network backend was > >> explicitly tested. > > > > I haven't had a chance to try this change with passt(1) yet (the > > backend can be enabled using "-net passt" or by starting it > > separately). > > > > Given that passt implements migration on its own (in deeper detail in > > some sense, as TCP connections are preserved if IP addresses match), I > > wonder if it this might affect or break it somehow. > > passt implements migration only with the vhost-user backend ("-netdev vhost-user") that is > not supported by netpass. All the vhost-* cannot be supported because netpass cannot catch > packets on the virtio queues. Thanks for having a look! On this point... right, hence my question in: https://lore.kernel.org/qemu-devel/20260131032700.12f27487@elisabeth/ that is, is there a plan to add vhost support *for netpass*, eventually? It looks like yes: https://lore.kernel.org/qemu-devel/CACLfguUZpT-3sj4C8G8e+LB5GHpBfE_HKLOhyZ9qYR8bgkTOCw@mail.gmail.com/ but I'm not sure I got it right (Cindy? Jason?). > passt with "-netdev stream" doesn't implement migration, but QEMU can be migrated with it > and all the connections are lost. So netpass will forward packets for connections that > will be broken. Realistically, I don't think anybody will ever try to migrate VMs using -netdev stream with passt, so I guess we don't really have to care about this (it might help with some protocols, probably make UDP usage a bit worse, waste a bit of bandwidth with TCP... but that's it). The only existing (known) user of passt's migration feature is KubeVirt, which switched to passt's vhost-user interface entirely. > "-netdev passt" is only some kind of wrapper on top of "-netdev stream" and "-netdev > vhost-user" that starts the passt backend by itself (rather than expecting it has been > started by the user). -- Stefano