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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id u12sm20756610wrt.50.2021.07.19.05.42.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Jul 2021 05:42:59 -0700 (PDT) Date: Mon, 19 Jul 2021 13:42:57 +0100 From: "Dr. David Alan Gilbert" To: Markus Armbruster Subject: Re: -only-migrate and the two different uses of migration blockers (was: spapr_events: Sure we may ignore migrate_add_blocker() failure?) Message-ID: References: <87tukvaejt.fsf@dusky.pond.sub.org> <87lf62ydow.fsf@dusky.pond.sub.org> <875yx6oabe.fsf@dusky.pond.sub.org> <87sg0amuuz.fsf_-_@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: <87sg0amuuz.fsf_-_@dusky.pond.sub.org> User-Agent: Mutt/2.0.7 (2021-05-04) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.469, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Juan Quintela , qemu-devel@nongnu.org, David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Markus Armbruster (armbru@redhat.com) wrote: > We appear to use migration blockers in two ways: > > (1) Prevent migration for an indefinite time, typically due to use of > some feature that isn't compatible with migration. > > (2) Delay migration for a short time. > > Option -only-migrate is designed for (1). It interferes with (2). > > Example for (1): device "x-pci-proxy-dev" doesn't support migration. It > adds a migration blocker on realize, and deletes it on unrealize. With > -only-migrate, device realize fails. Works as designed. > > Example for (2): spapr_mce_req_event() makes an effort to prevent > migration degrate the reporting of FWNMIs. It adds a migration blocker > when it receives one, and deletes it when it's done handling it. This > is a best effort; if migration is already in progress by the time FWNMI > is received, we simply carry on, and that's okay. However, option > -only-migrate sabotages the best effort entirely. That's interesting; it's the first time I've heard of anyone using it as 'best effort'. I've always regarded blockers as blocking. > While this isn't exactly terrible, it may be a weakness in our thinking > and our infrastructure. I'm bringing it up so the people in charge are > aware :) Thanks. It almost feels like they need a way to temporarily hold off 'completion' of migratio - i.e. the phase where we stop the CPU and write the device data; mind you you'd also probably want it to stop cold-migrates/snapshots? Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK