Re: [PATCH v4] failover: unregister ROM on unplug

[Laurent Vivier <lvivier@redhat.com>]

On 11/08/2021 08:50, Michael S. Tsirkin wrote:
> On Wed, Jul 21, 2021 at 06:09:05PM +0200, Laurent Vivier wrote:
>> The intend of failover is to allow to migrate a VM with a VFIO
>> networking card without disrupting the network operation by switching
>> to a virtio-net device during the migration.
>>
>> This simple change allows to test failover with a simulated device
>> like e1000e rather than a vfio device, even if it's useless in real
>> life it can help to debug failover.
>>
>> This is interesting to developers that want to test failover on
>> a system with no vfio device. Moreover it simplifies host networking
>> configuration as we can use the same bridge for virtio-net and
>> the other failover networking device.
>>
>> Without this change the migration of a system configured with failover
>> fails with:
>>
>>   ...
>>   -device virtio-net-pci,id=virtionet0,failover=on,...  \
>>   -device e1000,failover_pair_id=virtionet0,... \
>>   ...
>>
>>   (qemu) migrate ...
>>
>>   Unknown ramblock "0000:00:01.1:00.0/e1000e.rom", cannot accept migration
>>   error while loading state for instance 0x0 of device 'ram'
>>   load of migration failed: Invalid argument
>>
>> This happens because QEMU correctly unregisters the interface vmstate but
>> not the ROM one. This patch fixes that.
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> 
> I began to wonder about this. Why are we sending the option ROM at all?
> I think there's no need to do it for the primary ...

I think there is no way to check the ROM is the same on source and destination, and it has
to be.

By sending the ROM:
- we can check the size is the same, otherwise the migration fails,
- after the migration, even if a different ROM was provided on the destination side, the
guest executes the ROM provided by the source.

Thanks,
Laurent