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Wed, 13 May 2020 10:54:02 +0000 (UTC) Date: Wed, 13 May 2020 11:53:59 +0100 From: "Dr. David Alan Gilbert" To: Kevin Wolf Subject: Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu Message-ID: <20200513105359.GF3225@work-vm> References: <20200511114947.GJ1135885@redhat.com> <20200511120718.GD2811@work-vm> <20200511121714.GL1135885@redhat.com> <20200511154645.GI2811@work-vm> <20200512113206.62836e44@luklap> <20200512094337.GK1191162@redhat.com> <20200513103245.GD6202@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200513103245.GD6202@linux.fritz.box> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/13 03:05:18 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] 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_PASS=-0.001, T_HK_NAME_DR=0.01 autolearn=_AUTOLEARN 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: Lukas Straub , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , qemu-block , Juan Quintela , qemu-devel , Max Reitz , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Kevin Wolf (kwolf@redhat.com) wrote: > Am 12.05.2020 um 11:43 hat Daniel P. Berrangé geschrieben: > > On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote: > > > On Mon, 11 May 2020 16:46:45 +0100 > > > "Dr. David Alan Gilbert" wrote: > > > > > > > * Daniel P. Berrangé (berrange@redhat.com) wrote: > > > > > ... > > > > > That way if QEMU does get stuck, you can start by tearing down the > > > > > least distruptive channel. eg try tearing down the migration connection > > > > > first (which shouldn't negatively impact the guest), and only if that > > > > > doesn't work then, move on to tear down the NBD connection (which risks > > > > > data loss) > > > > > > > > I wonder if a different way would be to make all network connections > > > > register with yank, but then make yank take a list of connections to > > > > shutdown(2). > > > > > > Good Idea. We could name the connections (/yank callbacks) in the > > > form "nbd:", "chardev:" and "migration" > > > (and add "netdev:...", etc. in the future). Then make yank take a > > > list of connection names as you suggest and silently ignore connections > > > that don't exist. And maybe even add a 'query-yank' oob command returning > > > a list of registered connections so the management application can do > > > pattern matching if it wants. > > I'm generally not a big fan of silently ignoring things. Is there a > specific requirement to do it in this case, or can management > applications be expected to know which connections exist? > > > Yes, that would make the yank command much more flexible in how it can > > be used. > > > > As an alternative to using formatted strings like this, it could be > > modelled more explicitly in QAPI > > > > { 'struct': 'YankChannels', > > 'data': { 'chardev': [ 'string' ], > > 'nbd': ['string'], > > 'migration': bool } } > > > > In this example, 'chardev' would accept a list of chardev IDs which > > have it enabled, 'nbd' would accept a list of block node IDs which > > have it enabled, and migration is a singleton on/off. > > Of course, it also means that the yank code needs to know about every > single object that supports the operation, whereas if you only have > strings, the objects could keep registering their connection with a > generic function like yank_register_function() in this version. > > I'm not sure if the additional complexity is worth the benefits. I tend to agree; although we do have to ensure we either use an existing naming scheme (e.g. QOM object names?) or make sure we've got a well defined list of prefixes. Dave > > > The benefit of this modelling is that you can introspect QEMU > > to discover what classes of channels support being yanked by > > this QEMU build, as well as what instances are configured to > > be yanked. ie you can distinguish between a QEMU that doesn't > > support yanking network devices, from a QEMU that does support > > yanking network devices, but doesn't have it enabled for any > > network device instances. > > This is true, though I think Lukas' suggestion with query-yank should be > as good in practice (you can't check before creating the NBD device > then, but would you actually want to do this?). > > And if all else fails, we can still add a few more feature flags to the > schema... > > Kevin -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK