Re: [PATCHv2 RFC] virtio-spec: flexible configuration layout

Michael S. Tsirkin
2011-11-11T13:03:06Z

On Fri, Nov 11, 2011 at 02:54:31PM +1030, Rusty Russell wrote:
> On Wed, 09 Nov 2011 22:57:28 +0200, Sasha Levin <levinsasha928@gmail.com> wrote:
> > On Wed, 2011-11-09 at 22:52 +0200, Michael S. Tsirkin wrote:
> > > On Wed, Nov 09, 2011 at 10:24:47PM +0200, Sasha Levin wrote:
> > > > It'll be a bit harder deprecating it in the future.
> > >
> > > Harder than ... what ?
> >
> > Harder than allowing devices not to present it at all if new layout
> > config is used. Right now the simple implementation is to use MMIO for
> > config and device specific, and let it fallback to legacy for ISR and
> > notifications (and therefore, this is probably how everybody will
> > implement it), which means that when you do want to deprecate legacy,
> > there will be extra work to be done then, instead of doing it now.
>
> Indeed, I'd like to see two changes to your proposal:
>
> (1) It should be all or nothing. If a driver can find the virtio header
> capability, it should only use the capabilties. Otherwise, it
> should fall back to legacy. Your draft suggests a mix is possible;
> I prefer a clean failure (ie. one day don't present a BAR 0 *at
> all*, so ancient drivers just fail to load.).
> (2) There's no huge win in keeping the same layout. Let's make some
> cleanups. There are more users ahead of us then behind us (I
> hope!).
> But I think this is the right direction!
>
> Thanks,
> Rusty.

I'll do these changes, thanks!