Re: [PATCH 2/2] failover: don't allow to migrate a paused VM that needs PCI unplug

[Laurent Vivier]

On 02/11/2021 18:08, Michael S. Tsirkin wrote:
> On Tue, Nov 02, 2021 at 06:06:51PM +0100, Laurent Vivier wrote:
> > On 02/11/2021 16:04, Michael S. Tsirkin wrote:
> > > On Wed, Sep 29, 2021 at 04:43:11PM +0200, Laurent Vivier wrote:
> > > > As the guest OS is paused, we will never receive the unplug event
> > > > from the kernel and the migration cannot continue.
> > > >
> > > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > >
> > > Well ... what if user previously did
> > >
> > > pause
> > > start migration
> > > unpause
> > >
> > > we are breaking it now for no good reason.
> > >
> > > Further, how about
> > >
> > > start migration
> > > pause
> > >
> > > are we going to break this too? by failing pause?
> >
> > TL;DR: This patch only prevents to migrate a VFIO device as failover allows
> > to start a migration with a VFIO device plugged in.
> >
> > Long Story:
> >
> > * before this patch:
> >
> > - pause and start migration and unpause-> fails if we unpause too late
> > because we migrate a VFIO device, works otherwise
>
>
> confused about this one. can you explain pls?

Sorry, I've been confused by another bug: with ACPI unplug, we don't wait the unplug, and 
so if the machine is paused the VFIO is "migrated" and we have an error message on the 
destination side as the card cannot be plugged back.

but with PCIe native hotplug ("-global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=off") 
we have:

before this patch:

if we pause and then start the migration, migration hangs until we unpause the VM. But the 
migration can hangs forever if the VM is never unpaused. Normally migration of a paused VM 
should not hang.

after this patch:

if we pause and then start the migration, the migration fails because of the 
new error.

Remember that the migration of a VM with a VFIO device normally fails, so a user should 
not try to migrate a VM with a VFIO device except if he knows he is using failover, and in 
this case he should know he must not pause the VM.

Thanks,
Laurent