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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 19, 2018 at 06:07:07PM +0200, Paolo Bonzini wrote:
> On 19/04/2018 17:59, Michael S. Tsirkin wrote:
> > On Thu, Apr 19, 2018 at 05:51:51PM +0200, Paolo Bonzini wrote:
> >> 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.
> > 
> > Oh you are right. Well we can just special-case that one :)
> 
> Anything wrong with bumping the PCI revision?
> 
> Paolo

Nothing except in virtio 1 bumping the revision does not prevent
loading the old driver.

-- 
MST