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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f4635082csm17490482f8f.7.2026.07.15.08.48.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Jul 2026 08:48:43 -0700 (PDT) Date: Wed, 15 Jul 2026 11:48:40 -0400 From: "Michael S. Tsirkin" To: Michael Tokarev Cc: Vladimir Sementsov-Ogievskiy , jasowang@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 Subject: Re: [PATCH v19 00/15] virtio-net: live-TAP local migration Message-ID: <20260715114802-mutt-send-email-mst@kernel.org> References: <20260714154246.1242856-1-vsementsov@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Wed, Jul 15, 2026 at 06:01:39PM +0300, Michael Tokarev wrote: > On 14.07.2026 18:42, Vladimir Sementsov-Ogievskiy wrote: > > Hi all! > > > > Here is a migration for TAP net backend, including its properties and > > open fds. > > > > With this new feature, management software doesn't need to initialize > > new TAP and do a switch to it. Nothing should be done around > > virtio-net in local migration: it just migrates and continues to use > > same TAP device. So we avoid extra logic in management software, extra > > allocations in kernel (for new TAP), and corresponding extra delay in > > migration downtime. > > This is quite a big patch set, - is this really worth the effort to do > all this just for *local* migration? What's the possible use case for > this in real, - am I right this is just about upgrading the host qemu? > And with that in mind, isn't it sufficient to use what we already have > (namely, create new tap, start new qemu instance, and migrate the usual > way), and tolerate some very minor downtime while the networking code > learns the new network topology (isn't it happening almost instantly > anyway, and if not, the management can help by sending gratitious ARP)? > > I wonder what's the use for this at yandex? > > Thanks, > > /mjt Well just theoretically, imagine a big VM, reserving twice the amount of memory just to migrate is not nice at all.