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

[Laurent Vivier <lvivier@redhat.com>]

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
- start migration and pause before we unplug the card -> hangs forever
- start migration and pause after we unplug the card -> it works fine

* After this patch:

- pause and start migration and unpause-> fails if we unpause too late because of the new 
error checking, works otherwise
- start migration and pause before we unplug the card -> fails because of the new error 
checking
- start migration and pause after we unplug the card -> it works fine

Thanks,
Laurent