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Wed, 13 Aug 2025 02:25:36 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEJHxy2+ELPRV2u1frzgzrSyzqIyrFVKk4w2ejulkmmasmh8wC+ctbInR3L9ay8X8gNKQZyH8PxNFc8IazZH5Q= X-Received: by 2002:a17:903:124e:b0:237:d734:5642 with SMTP id d9443c01a7336-2430d26ec95mr33124045ad.41.1755077136381; Wed, 13 Aug 2025 02:25:36 -0700 (PDT) MIME-Version: 1.0 References: <20250722124127.2497406-1-jonah.palmer@oracle.com> <20250722124127.2497406-6-jonah.palmer@oracle.com> <5e276607-cd86-4a1d-99f3-47dd2f0f3bc0@oracle.com> <0cf99747-443e-4a29-a0da-64012548a994@oracle.com> In-Reply-To: From: Eugenio Perez Martin Date: Wed, 13 Aug 2025 11:25:00 +0200 X-Gm-Features: Ac12FXwlLOQoxyIZqRy6CLxY5KmZ2k5PshqO27m8qA5tqsTgEHJJnj-S3Zsfv3Y Message-ID: Subject: Re: [RFC 5/6] virtio,virtio-net: skip consistency check in virtio_load for iterative migration To: Peter Xu Cc: Jonah Palmer , qemu-devel@nongnu.org, farosas@suse.de, eblake@redhat.com, armbru@redhat.com, jasowang@redhat.com, mst@redhat.com, si-wei.liu@oracle.com, boris.ostrovsky@oracle.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=170.10.129.124; envelope-from=eperezma@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_H5=0.001, RCVD_IN_MSPIKE_WL=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: 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 Mon, Aug 11, 2025 at 11:56=E2=80=AFPM Peter Xu wrote= : > > On Mon, Aug 11, 2025 at 05:26:05PM -0400, Jonah Palmer wrote: > > This effort was started to reduce the guest visible downtime by > > virtio-net/vhost-net/vhost-vDPA during live migration, especially > > vhost-vDPA. > > > > The downtime contributed by vhost-vDPA, for example, is not from having= to > > migrate a lot of state but rather expensive backend control-plane laten= cy > > like CVQ configurations (e.g. MQ queue pairs, RSS, MAC/VLAN filters, of= fload > > settings, MTU, etc.). Doing this requires kernel/HW NIC operations whic= h > > dominates its downtime. > > > > In other words, by migrating the state of virtio-net early (before the > > stop-and-copy phase), we can also start staging backend configurations, > > which is the main contributor of downtime when migrating a vhost-vDPA > > device. > > > > I apologize if this series gives the impression that we're migrating a = lot > > of data here. It's more along the lines of moving control-plane latency= out > > of the stop-and-copy phase. > > I see, thanks. > > Please add these into the cover letter of the next post. IMHO it's > extremely important information to explain the real goal of this work. I > bet it is not expected for most people when reading the current cover > letter. > > Then it could have nothing to do with iterative phase, am I right? > > What are the data needed for the dest QEMU to start staging backend > configurations to the HWs underneath? Does dest QEMU already have them i= n > the cmdlines? > > Asking this because I want to know whether it can be done completely > without src QEMU at all, e.g. when dest QEMU starts. > > If src QEMU's data is still needed, please also first consider providing > such facility using an "early VMSD" if it is ever possible: feel free to > refer to commit 3b95a71b22827d26178. > While it works for this series, it does not allow to resend the state when the src device changes. For example, if the number of virtqueues is modified. > So the data to be transferred is still in VMSD form, aka, data are still > described by VMSD macros, instead of hard-coded streamline protocols usin= g > e.g. qemufile APIs using save_setup()/load_setup(). > > When things are described in VMSDs, it get the most benefit from the live > migration framework, and it's much, much more flexible. It's the most > suggested way for device to cooperate with live migration, savevmhandlers > are only the last resort because it's almost not in control of migration.= . > > In short, please avoid using savevmhandlers as long as there can be any > other way to achieve similar results. > > Thanks, > > -- > Peter Xu >