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Tue, 20 Jul 2021 07:30:16 +0200 (CEST) From: Markus Armbruster To: "Dr. David Alan Gilbert" Subject: Re: -only-migrate and the two different uses of migration blockers 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> Date: Tue, 20 Jul 2021 07:30:16 +0200 In-Reply-To: (David Alan Gilbert's message of "Mon, 19 Jul 2021 13:42:57 +0100") Message-ID: <87o8axh7rr.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@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" "Dr. David Alan Gilbert" writes: > * 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. Me too, until I found this one. >> 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? Yes, a proper way to delay 'completion' for a bit would be clearer, and wouldn't let -only-migrate interfere.