* Jens Freimann (jfreim...@redhat.com) wrote: > On Tue, Jun 04, 2019 at 08:00:19PM +0100, Dr. David Alan Gilbert wrote: > > * Michael S. Tsirkin (m...@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 (jfreim...@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?
Potentially; lets see what the libvirt people have to say. What happens if you have multiple devices and one of them unplugs OK and then the other fails? > 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. Dave > regards, > Jens -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
