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

[Michael S. Tsirkin]

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.
> 
> Thanks,
> 
> Paolo

Oh you are right. Well we can just special-case that one :)

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MST