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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id c125sm3660314wme.36.2021.07.22.11.00.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Jul 2021 11:00:58 -0700 (PDT) Date: Thu, 22 Jul 2021 19:00:56 +0100 From: "Dr. David Alan Gilbert" To: David Gibson Subject: Re: -only-migrate and the two different uses of migration blockers 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> <87o8axh7rr.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: 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=-1.472, 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 , Markus Armbruster , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * David Gibson (david@gibson.dropbear.id.au) wrote: > On Tue, Jul 20, 2021 at 07:30:16AM +0200, Markus Armbruster wrote: > > "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. > > Right, it may well have been the first usage this way, this fwnmi > stuff isn't super old. > > > >> 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. > > Right. If that becomes a thing, we should use it here. Note that > this one use case probably isn't a very strong argument for it, > though. The only problem here is slightly less that optimal error > reporting in a rare edge case (hardware fault occurs by chance at the > same time as a migration). Can you at least put a scary comment in to say why it's so odd. If you wanted a choice of a different bad way to do this, since you have savevm_htab_handlers, you might be able to make htab_save_iterate claim there's always more to do. > > .... and, also, I half-suspect that the whole fwnmi feature exists > more to tick IBM RAS check boxes than because anyone will actually use > it. Ah at least it's always reliable.... Dave > -- > David Gibson | I'll have my music baroque, and my code > david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ > | _way_ _around_! > http://www.ozlabs.org/~dgibson -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK