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Tsirkin" To: Anton Kuchin Cc: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, yc-core@yandex-team.ru, "Dr. David Alan Gilbert" , Markus Armbruster , Eduardo Habkost , Juan Quintela , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Stefan Hajnoczi , Paolo Bonzini , virtio-fs@redhat.com, Eric Blake Subject: Re: [PATCH v3 1/1] vhost-user-fs: add migration type property Message-ID: <20230222151814-mutt-send-email-mst@kernel.org> References: <20230217170038.1273710-2-antonkuchin@yandex-team.ru> <59165bde-bfd4-a073-c618-205be3951e4a@yandex-team.ru> <20230222074214-mutt-send-email-mst@kernel.org> <22fee9b6-1dc0-792c-13cf-54c9303556ab@yandex-team.ru> <8372550f-def7-4336-c597-d22155abf145@yandex-team.ru> <333c4451-8eef-0603-c3f5-10e38c0eb24e@yandex-team.ru> <20230222115106-mutt-send-email-mst@kernel.org> <11593688-7ca4-def3-6212-7c26faa4d1c6@yandex-team.ru> <20230222121133-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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: -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_H2=-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 Wed, Feb 22, 2023 at 08:25:19PM +0200, Anton Kuchin wrote: > On 22/02/2023 19:12, Michael S. Tsirkin wrote: > > On Wed, Feb 22, 2023 at 07:05:47PM +0200, Anton Kuchin wrote: > > > On 22/02/2023 18:51, Michael S. Tsirkin wrote: > > > > On Wed, Feb 22, 2023 at 06:49:10PM +0200, Anton Kuchin wrote: > > > > > On 22/02/2023 17:14, Vladimir Sementsov-Ogievskiy wrote: > > > > > > On 22.02.23 17:25, Anton Kuchin wrote: > > > > > > > > > > +static int vhost_user_fs_pre_save(void *opaque) > > > > > > > > > > +{ > > > > > > > > > > +    VHostUserFS *fs = opaque; > > > > > > > > > > +    g_autofree char *path = object_get_canonical_path(OBJECT(fs)); > > > > > > > > > > + > > > > > > > > > > +    switch (fs->migration_type) { > > > > > > > > > > +    case VHOST_USER_MIGRATION_TYPE_NONE: > > > > > > > > > > +        error_report("Migration is blocked by device %s", path); > > > > > > > > > > +        break; > > > > > > > > > > +    case VHOST_USER_MIGRATION_TYPE_EXTERNAL: > > > > > > > > > > +        return 0; > > > > > > > > > > +    default: > > > > > > > > > > +        error_report("Migration type '%s' is not > > > > > > > > > > supported by device %s", > > > > > > > > > > + VhostUserMigrationType_str(fs->migration_type), path); > > > > > > > > > > +        break; > > > > > > > > > > +    } > > > > > > > > > > + > > > > > > > > > > +    return -1; > > > > > > > > > > +} > > > > > > > > > Should we also add this as .pre_load, to force user select > > > > > > > > > correct migration_type on target too? > > > > > > > > In fact, I would claim we only want pre_load. > > > > > > > > When qemu is started on destination we know where it's migrated > > > > > > > > from so this flag can be set. > > > > > > > > When qemu is started on source we generally do not yet know so > > > > > > > > we don't know whether it's safe to set this flag. > > > > > > But destination is a "source" for next migration, so there shouldn't be > > > > > > real difference. > > > > > > The new property has ".realized_set_allowed = true", so, as I understand > > > > > > it may be changed at any time, so that's not a problem. > > > > > Yes, exactly. So destination's property sets not how it will handle this > > > > > incoming > > > > > migration but the future outgoing one. > > > > How do you know where you are going to migrate though? > > > > I think you don't. > > > > Setting it on source is better since we know where we > > > > are migrating from. > > > Yes, I don't know where I'm going to migrate to. This is why property > > > affects only how source saves state on outgoing migration. > > Um. I don't get the logic. > > For this feature to work we need orchestrator to manage the migration. And > we > generally assume that it is responsibility of orchestrator to ensure > matching > properties on source and destination. > As orchestrator manages both sides of migration it can set option (and we > can > check it) on either source or destination. Now it's not important which side > we > select, because now the option is essentially binary allow/deny (but IMHO it > is much better to refuse source to migrate than find later that state can't > be > loaded by destination, in case of file migration this becomes especially > painful). > > But there are plans to add internal migration option (extract FUSE state > from > backend and transfer it in QEMU migration stream), and that's where > setting/checking > on source becomes important because it will rely on this property to decide > if > extra state form backend needs to be put in the migration stream subsection. If we do internal migration that will be a different property which has to match on source *and* destination. > If you are concerned about orchestrator breaking assumption of matching > properties > on source and destination this is not really supported AFAIK but I don't > think we > need to punish it for this, maybe it has its reasons: I can imagine scenario > where orchestrator could want to migrate from source with > 'migration=external' > to destination with 'migration=none' to ensure that destination can't be > migrated further. No. I am concerned about a simple practical matter: - I decide to restart qemu on the same host - so I need to enable migration - Later I decide to migrate qemu to another host - this should be blocked Property on source does not satisfy both at the same time. Property on destination does. > > > > > > > > > > > This property selects if VM can migrate and if it can what should > > > > > > > qemu put > > > > > > > to the migration stream. So we select on source what type of > > > > > > > migration is > > > > > > > allowed for this VM, destination can't check anything at load time. > > > > > > OK, so the new field "migration" regulates only outgoing migration and > > > > > > do nothing for incoming. On incoming migration the migration stream > > > > > > itself defines the type of device migration. > > > > > > Worth mentioning in doc? > > > > > Good point. I don't think this deserves a respin but if I have to send v4 > > > > > I'll include > > > > > clarification in it.