Re: [Qemu-devel] [virtio-dev] RE: [PATCH v3 6/6] vhost-user: support registering external host notifiers

[Paolo Bonzini]

On 19/04/2018 17:19, Michael S. Tsirkin wrote:
>> - if we make it 1 when weak barriers are needed, the device also needs
>> to nack feature negotiation (not allow setting the FEATURES_OK) if the
>> bit is not set by the driver.
>>  However, that is not enough.  Live
>> migration assumes that it is okay to migrate a virtual machine from a
>> source that doesn't support a feature to a destination that supports it.
>>  In this case, it would assume that it is okay to migrate from software
>> virtio to hardware virtio.  This is wrong because the destination would
>> use weak barriers
> 
> You can't migrate between systems with different sets of device features
> right now.

Yes, you can, exactly because some features are defined not by the
machine type but rather by the host kernel.  See virtio_net_get_features
in QEMU's hw/virtio/virtio-net.c, and virtio_set_features_nocheck in
QEMU's hw/virtio/virtio.c.

Thanks,

Paolo