Re: [PATCH RFC] virtio-spec: flexible configuration layout

Michael S. Tsirkin
2011-11-09T15:18:59Z

On Wed, Nov 09, 2011 at 02:48:58PM +0200, Sasha Levin wrote:
> On Wed, 2011-11-09 at 14:38 +0200, Avi Kivity wrote:
> > On 11/09/2011 10:46 AM, Sasha Levin wrote:
> > > > Alternatively we can add new structures with new
> > > > structure IDs, pointed to from PCI configuration space.
> > > >
> > > > As we don't yet have devices or drivers with 64 bit features,
> > > > I decided we don't need high feature bits in legacy space.
> > > > This also frees up feature bit 31 as we don't need it
> > > > to enable high feature bits anymore.
> > >
> > > KVM tool actually has support for 64bit features, we can probably remove
> > > that when Pekka isn't looking :)
> > >
> >
> > What about the Windows drivers?
>
> While 64-bit features were defined, there was no actual devices to use
> them.
>
> So even if Windows drivers had the ability to support 64-bit features,
> no device ever needed it therefore no device ever activated it.
>
> A potential issue might arise when we remap feature bit 31 to do
> something else, and when activated it would fool Windows drivers to
> think that 64-bit features are now activated, while they're not - but
> thats not something that should happen since legacy layout shouldn't
> have that field enabled as per the spec (no new features in legacy
> code).
>
> I was unable to check if it was actually implemented in the drivers
> because
> http://git.kernel.org/?p=virt/kvm/kvm-guest-drivers-windows.git;a=summary is not quite there (*cough*).

I'm pretty sure windows drivers didn't yet get to it.
This was out for a very short time.

> --
>
> Sasha.
>