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Re: [Qemu-devel] [PATCH 1/4] migration: allow unplug during migration fo

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH 1/4] migration: allow unplug during migration for failover devices
Date: Thu, 23 May 2019 16:37:58 +0100
User-agent: Mutt/1.11.4 (2019-03-13)

* Jens Freimann (address@hidden) wrote:
> On Tue, May 21, 2019 at 10:33:36AM +0100, Dr. David Alan Gilbert wrote:
> > * Jens Freimann (address@hidden) wrote:
> > > In "b06424de62 migration: Disable hotplug/unplug during migration" we
> > > added a check to disable unplug for all devices until we have figured
> > > out what works. For failover primary devices qdev_unplug() is called
> > > from the migration handler, i.e. during migration.
> > > 
> > > This patch adds a flag to DeviceState which is set to false for all
> > > devices and makes an exception for vfio-pci devices that are also
> > > primary devices in a failover pair.
> > > 
> > > Signed-off-by: Jens Freimann <address@hidden>
> > 
> > So I think this is safe in your case, because you trigger the unplug
> > right at the start of migration during setup and plug after failure;
> > however it's not generally safe - I can't unplug a device while the
> > migration is actually in progress.
> I tried to limit it to only allow it in failover case. You're saying
> it's missing something and not strict enough? I could allow it only
> during migration setup. I guess we'll need a similar exception for
> failover in libvirt.

I might be wrong, but I think with your patch I could hot unplug your
device part way through migration; where as I think you only care about
it doing it at a very specific point during setup.

(I still would prefer the hotplug to be done outside qemu, but still
that's separate).

> regards,
> Jens
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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