Re: [Qemu-devel] [2.5 issue] virtio-1 in virtio-net and old vhost

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

On Fri, Dec 04, 2015 at 11:15:48AM +0100, Cornelia Huck wrote:
> On Fri, 4 Dec 2015 10:06:58 +0800
> Jason Wang <jasowang@redhat.com> wrote:
> 
> > The problem is for pci: without this patch, guest may always see modern
> > bar is "disable-modern=false". But with this patch, on an old kernel
> > that does not support VERSION_1, even "disable-modern=false" were
> > specified, guest can not see modern bar anymore. Looks like a guest
> > visible change.
> 
> But the guest even see a modern bar if the host is not able to present
> a spec-compliant device?
> 
> What we have now is a device that looks like a modern device but that
> does not offer VERSION_1. Probably not spec compliant. If virtio-pci is
> fixed to not present such broken devices to the guest, this is
> guest-visible, yes; but only in the sense that the guest will no longer
> see broken devices, but simply legacy devices if configured.

I agree - we don't have to keep what's broken, broken.
If guests are known not to work, we don't worry about
how well they migrate.

-- 
MST