Re: [virtio-comment] virtio and endian-ness

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

On Tue, Aug 20, 2013 at 10:23:13AM +0100, Pawel Moll wrote:
> On Tue, 2013-08-20 at 05:34 +0100, Rusty Russell wrote:
> > Yes, I think virtio-ccw and virtio-mmio could simply add a new feature
> > bit to say "1.0 compliant". 
> 
> MMIO devices have "version" register. Currently it's 1. The OASIS spec
> will bump it to 2 (main change will be the addressing scheme). So the
> guest drivers may support both versions.
> 
> Paweł

No, updating revision is a way to disable compatibility
in drivers.

So old drivers currently do:
if revision != 1 -> exit
new drivers will do
if revision < 1 || revision > 2 -> exit

Now a host can disable loading old drivers
by setting revision to 2.

However this must not be required by spec,
it must be possible to write a device that
implements both old and new spec.


I'll have to look at how mmio and ccw do
feature bits to figure out if a feature bit
will work for this.


-- 
MST