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[100.0.197.103]) by smtp.gmail.com with ESMTPSA id d123sm1210136qkb.94.2019.06.07.07.32.12 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Fri, 07 Jun 2019 07:32:13 -0700 (PDT) Date: Fri, 7 Jun 2019 10:32:06 -0400 From: "Michael S. Tsirkin" To: Jens Freimann Message-ID: <20190607103059-mutt-send-email-mst@kernel.org> References: <20190517125820.2885-4-jfreimann@redhat.com> <20190521094504.GB2915@work-vm> <20190530145645.tjwkgi4hae5yblsi@jenstp.localdomain> <20190531214748.GN22103@habkost.net> <20190603082456.vzpy256kj4o5e5wu@jenstp.localdomain> <20190603193648.GQ22103@habkost.net> <20190604134321.txlw7wjwe247g5ug@jenstp.localdomain> <20190604125037-mutt-send-email-mst@kernel.org> <20190604190019.GM3851@work-vm> <20190607141407.73hzf5w6vqjegaja@jenstp.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190607141407.73hzf5w6vqjegaja@jenstp.localdomain> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.160.193 Subject: Re: [Qemu-devel] [PATCH 3/4] net/virtio: add failover support 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: pkrempa@redhat.com, berrange@redhat.com, Eduardo Habkost , aadam@redhat.com, qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , laine@redhat.com, ailan@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jun 07, 2019 at 04:14:07PM +0200, Jens Freimann wrote: > On Tue, Jun 04, 2019 at 08:00:19PM +0100, Dr. David Alan Gilbert wrote: > > * Michael S. Tsirkin (mst@redhat.com) wrote: > > > On Tue, Jun 04, 2019 at 03:43:21PM +0200, Jens Freimann wrote: > > > > On Mon, Jun 03, 2019 at 04:36:48PM -0300, Eduardo Habkost wrote: > > > > > On Mon, Jun 03, 2019 at 10:24:56AM +0200, Jens Freimann wrote: > > > > > > On Fri, May 31, 2019 at 06:47:48PM -0300, Eduardo Habkost wrote: > > > > > > > On Thu, May 30, 2019 at 04:56:45PM +0200, Jens Freimann wrote: > > > > > > > > On Tue, May 28, 2019 at 11:04:15AM -0400, Michael S. Tsirkin wrote: > > > > > > > > > On Tue, May 21, 2019 at 10:45:05AM +0100, Dr. David Alan Gilbert wrote: > > > > > > > > > > * Jens Freimann (jfreimann@redhat.com) wrote: > > > > > > Why is it bad to fully re-create the device in case of a failed migration? > > > > > > > > > > Bad or not, I thought the whole point of doing it inside QEMU was > > > > > to do something libvirt wouldn't be able to do (namely, > > > > > unplugging the device while not freeing resources). If we are > > > > > doing something that management software is already capable of > > > > > doing, what's the point? > > > > > > > > Event though management software seems to be capable of it, a failover > > > > implementation has never happened. As Michael says network failover is > > > > a mechanism (there's no good reason not to use a PT device if it is > > > > available), not a policy. We are now trying to implement it in a > > > > simple way, contained within QEMU. > > > > > > > > > Quoting a previous message from this thread: > > > > > > > > > > On Thu, May 30, 2019 at 02:09:42PM -0400, Michael S. Tsirkin wrote: > > > > > | > On Thu, May 30, 2019 at 07:00:23PM +0100, Dr. David Alan Gilbert wrote: > > > > > | > > This patch series is very > > > > > | > > odd precisely because it's trying to do the unplug itself in the > > > > > | > > migration phase rather than let the management layer do it - so unless > > > > > | > > it's nailed down how to make sure that's really really bullet proof > > > > > | > > then we've got to go back and ask the question about whether we should > > > > > | > > really fix it so it can be done by the management layer. > > > > > | > > > > > > > | > > Dave > > > > > | > > > > > > | > management already said they can't because files get closed and > > > > > | > resources freed on unplug and so they might not be able to re-add device > > > > > | > on migration failure. We do it in migration because that is > > > > > | > where failures can happen and we can recover. > > > > > > > > This is something that I can work on as well, but it doesn't have to > > > > be part of this patch set in my opinion. Let's say migration fails and we can't > > > > re-plug the primary device. We can still use the standby (virtio-net) > > > > device which would only mean slower networking. How likely is it that > > > > the primary device is grabbed by another VM between unplugging and > > > > migration failure anyway? > > > > > > > > regards, > > > > Jens > > > > > > I think I agree with Eduardo it's very important to handle this corner > > > case correctly. Fast networking outside migration is why people use > > > failover at all. Someone who can live with a slower virtio would use > > > just that. > > > > > > And IIRC this corner case is exactly why libvirt could not > > > implement it correctly itself and had to push it up the stack > > > until it fell off the cliff :). > > > > So I think we need to have the code that shows we can cope with the > > corner cases - or provide a way for libvirt to handle it (which is > > my strong preference). > > Would this work: We add a new migration state MIGRATE_WAIT_UNPLUG (or > a better more generic name) which tells libvirt that migration has not > started yet because we are waiting for the guest. And extend the qmp > events for the migration state. When we know the device was > sucessfully unplugged we sent a qmp event DEVICE_DELETED or a new one > DEVICE_DELETED_PARTIALLY (not sure about that yet), let migration > start and set the migration state to active? > > To do a partial unplug I imagine, we have to separate vfio(-pci) code > to differ between release of resources (fds, mappings etc) and unplug > (I haven't yet found out how it works in vfio). In the failover case > we only do the unplug part but not the release part. > > regards, > Jens I think the first is done in vfio_exitfn and the second in vfio_instance_finalize. -- MST