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[80.230.24.117]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47f4634e029sm19300702f8f.3.2026.07.15.13.32.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Jul 2026 13:32:21 -0700 (PDT) Date: Wed, 15 Jul 2026 16:32:18 -0400 From: "Michael S. Tsirkin" To: Vladimir Sementsov-Ogievskiy Cc: Peter Xu , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Michael Tokarev , jasowang@redhat.com, armbru@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 Subject: Re: [PATCH v19 00/15] virtio-net: live-TAP local migration Message-ID: <20260715163116-mutt-send-email-mst@kernel.org> References: <20260714154246.1242856-1-vsementsov@yandex-team.ru> <20260715114802-mutt-send-email-mst@kernel.org> <142f9286-bbc7-4aed-8822-5d560a09daf9@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: <142f9286-bbc7-4aed-8822-5d560a09daf9@yandex-team.ru> Received-SPF: permerror client-ip=170.10.133.124; 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_H3=-0.01, RCVD_IN_MSPIKE_WL=-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 11:21:23PM +0300, Vladimir Sementsov-Ogievskiy wrote: > On 15.07.26 19:00, Peter Xu wrote: > > On Wed, Jul 15, 2026 at 04:52:48PM +0100, Daniel P. Berrangé wrote: > > > On Wed, Jul 15, 2026 at 11:48:40AM -0400, Michael S. Tsirkin wrote: > > > > 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. > > > > > > Don't we already have the ability to skip memory transfer by setting > > > the "x-ignore-shared" capability, assuming the VM RAM has a shared > > > memory backing. > > > > Right, IIUC all similar single-host migrations like this series or CPR (or > > anything else...) should always need to enable x-ignore-shared in the first > > place. That's almost always the starting point of optimizing local > > migrations.. no matter how the memory will be shared (by the same pool of > > page cache, or persisted over kexec, etc.). > > > > Yes, sharing RAM between source and target + enabling x-ignore-shared is a first thing to do. > > This series optimizes TAP recreating. Not only skip recreating but also allow to > exclude cloud-networking component form live-update entirely, making the process > simpler (less components involved), and as I already said, reducing corresponding > downtime. > > -- > Best regards, > Vladimir So can you explain, how is this better than 1. a persistent tap 2. a non persistent tap that some server gets a hold of IOW why does qemu need to bother. -- MST