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Tsirkin" To: Akihiko Odaki Cc: Peter Xu , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Thomas Huth , Yuri Benditovich , eduardo@habkost.net, marcel.apfelbaum@gmail.com, philmd@linaro.org, wangyanan55@huawei.com, dmitry.fleytman@gmail.com, jasowang@redhat.com, sriram.yagnaraman@est.tech, sw@weilnetz.de, qemu-devel@nongnu.org, yan@daynix.com, Fabiano Rosas , devel@lists.libvirt.org Subject: Re: [PATCH v2 4/4] virtio-net: Add support for USO features Message-ID: <20240808065339-mutt-send-email-mst@kernel.org> References: <8ad96f43-83ae-49ae-abc1-1e17ee15f24d@daynix.com> <20240805032937-mutt-send-email-mst@kernel.org> <20240805041650-mutt-send-email-mst@kernel.org> <20240805060544-mutt-send-email-mst@kernel.org> <2b62780c-a6cb-4262-beb5-81d54c14f545@daynix.com> <20240806092822-mutt-send-email-mst@kernel.org> <890f9d0a-3ded-488d-b274-8be9c38b5df3@daynix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <890f9d0a-3ded-488d-b274-8be9c38b5df3@daynix.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.141, 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=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=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 Thu, Aug 08, 2024 at 07:52:37PM +0900, Akihiko Odaki wrote: > On 2024/08/06 22:29, Michael S. Tsirkin wrote: > > On Tue, Aug 06, 2024 at 04:35:44PM +0900, Akihiko Odaki wrote: > > > On 2024/08/05 19:08, Michael S. Tsirkin wrote: > > > > On Mon, Aug 05, 2024 at 06:37:58PM +0900, Akihiko Odaki wrote: > > > > > If cross-migrate=off, QEMU can still migrate on the same host (checkpoint > > > > > and restart). QEMU can also migrate across hosts if the user ensures they > > > > > are on the same platform. > > > > > > > > What is so special about checkpoint/restart? I guess we hope that > > > > downgrades are uncommon, but they are possible... > > > > > > Downgrades will not work with cross-migrate=off. Users who want downgrades > > > should use cross-migrate=on. > > > > We also don't know that upgrades do not disable a feature: > > can happen if e.g. there's a serious bug in the feature. > > Basically, this makes the feature too fragile, in my opinion. > > We can do nothing in such a case. Whether it is on a single host or multiple > hosts, we cannot support migration if features once enabled disappear. > > Regards, > Akihiko Odaki It does not follow that we have to do something, and this is something, therefore that we have to do this. This is just a reason not to handle checkpoint/restart any different than any other migration. -- MST