Re: [virtio] Re: [virtio-comment] [PATCH RFC] virtio-pci: new configuration layout

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

On Mon, Sep 02, 2013 at 10:45:11AM +0930, Rusty Russell wrote:
> > More concerns:
> >
> > We are using revision ID now exactly as was intended to disable old
> > drivers - it served us well for 0.X-1.X and would be as useful if we
> > ever have 1.X->2.0 transition.
> 
> I think we'll use feature bit 64 for such a transition, now we don't
> have a feature bit limit.

Two issues:

1. Basically on windows this would interfere with the idea of using
different drivers for legacy and non-legacy devices
http://msdn.microsoft.com/en-us/library/windows/hardware/ff549553%28v=vs.85%29.aspx
windows build up device identification 

2. We'd have to declare this use for a feature bit right now,
make current drivers check this bit and bail out if set.