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

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

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.

Okay, but going forward, if we add more capabilities, we probably won't
want to require them and fail to load if not there.  That's really why I
wanted to make the failover ignore any capability separately - to make
this future proof.  I'm not terribly fixated on this, it just seemed a
bit more symmetrical to treat all capabilities in the same way. Hmm?

>     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.).

Just to clarify, as written in draft this is possible with the current
spec proposal.  So I'm guessing there's some other motivation that you
had in mind?

> (2) There's no huge win in keeping the same layout.  Let's make some
>     cleanups.

About this last point - what cleanups do you have in mind?  Just move
some registers around?  I guess we could put feature bits near each
other, and move device status towards the end to avoid wasting 3 bytes.
The win seems minimal, but the change does make legacy hypervisor
support in guests more cumbersome, as we need to spread coditional code
around instead of localizing it in the initialization path.

>    There are more users ahead of us then behind us (I
>     hope!).

In that case isn't it safe to assume we'll find some uses
for the reserved registers?

> But I think this is the right direction!
> 
> Thanks,
> Rusty.