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Tsirkin" , Stefano Garzarella , Kevin Wolf , Hanna Reitz , Fabiano Rosas , Eric Blake , Markus Armbruster , Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Subject: Re: [PATCH v3 0/3] vhost-user-blk: support inflight migration Message-ID: References: <20251110103937.1944486-1-dtalexundeer@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@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=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_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 Tue, Nov 18, 2025 at 11:24:12PM +0300, Vladimir Sementsov-Ogievskiy wrote: > Add Daniel > > On 10.11.25 13:39, Alexandr Moshkov wrote: > > v3: > > - use pre_load_errp instead of pre_load in vhost.c > > - change vhost-user-blk property to > > "skip-get-vring-base-inflight-migration" > > - refactor vhost-user-blk.c, by moving vhost_user_blk_inflight_needed() higher > > > > v2: > > - rewrite migration using VMSD instead of qemufile API > > - add vhost-user-blk parameter instead of migration capability > > > > I don't know if VMSD was used cleanly in migration implementation, so > > feel free for comments. > > > > Based on Vladimir's work: > > [PATCH v2 00/25] vhost-user-blk: live-backend local migration > > which was based on: > > - [PATCH v4 0/7] chardev: postpone connect > > (which in turn is based on [PATCH 0/2] remove deprecated 'reconnect' options) > > - [PATCH v3 00/23] vhost refactoring and fixes > > - [PATCH v8 14/19] migration: introduce .pre_incoming() vmsd handler > > > > Hi! > > On my series about backend-transfer migration, the final consensus (or at least, > I hope that it's a consensus:) is that using device properties to control migration > channel content is wrong. And we should instead use migration parameters. > > (discussion here: https://lore.kernel.org/qemu-devel/29aa1d66-9fa7-4e44-b0e3-2ca26e77accf@yandex-team.ru/ ) > > So the API for backend-transfer features is a migration parameter > > backend-transfer = [ list of QOM paths of devices, for which we want to enable backend-transfer ] > > and user don't have to change device properties in runtime to setup the following migration. > > So I assume, similar practice should be applied here: don't use device > properties to control migration. > > So, should it be a parameter like > > migrate-inflight-region = [ list of QOM paths of vhost-user devices ] > > ? I have concern that if we start doing this more, migration qapi/ will be completely messed up. Imagine a world where there'll be tons of lists like: migrate-dev1-some-feature-1 = [list of devices (almost only dev1 typed)] migrate-dev2-some-feature-2 = [list of devices (almost only dev2 typed)] migrate-dev3-some-feature-3 = [list of devices (almost only dev3 typed)] ... That doesn't look reasonable at all. If some feature is likely only supported in one device, that should not appear in migration.json but only in the specific device. I don't think I'm fully convinced we can't enable some form of machine type properties (with QDEV or not) on backends we should stick with something like that. I can have some closer look this week, but.. even if not, I still think migration shouldn't care about some specific behavior of a specific device. If we really want to have some way to probe device features, maybe we should also think about a generic interface (rather than "one new list every time"). We also have some recent discussions on a proper interface to query TAP backend features like USO*. Maybe they share some of the goals here. Thanks, -- Peter Xu