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Tsirkin" Cc: Peter Xu , =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= , 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 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> <20240808065339-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Akihiko Odaki In-Reply-To: <20240808065339-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: none client-ip=2607:f8b0:4864:20::633; envelope-from=akihiko.odaki@daynix.com; helo=mail-pl1-x633.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_NONE=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 2024/08/08 19:54, Michael S. Tsirkin wrote: > 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. Whethere it is checkpoint/restart or any other migration, I expect platform features won't disappear from the host(s); we can't readily support migration in such a situation. When platform features won't disappear, for checkpoint/restart, we can enable all available features without disrupting migration; cross-migrate=off will instruct that. However, if we are migrating a VM across hosts and the user doesn't ensure they are on the same platform, we cannot enable platform features even if we are sure that platform features already present on a host won't disappear because some hosts may not have features in the first place. We can set cross-migrate=on in such a case to disable optional platform features. Regards, Akihiko Odaki