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

[Rusty Russell <rusty@rustcorp.com.au>]

On Sun, 13 Nov 2011 17:14:27 +0200, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Fri, Nov 11, 2011 at 02:54:31PM +1030, Rusty Russell wrote:
> > 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?

Sure, a future capbility may not exist.  But once a driver finds that
virtio header structure in the capability, it should *never* fall back
to the legacy area.  ie. it can expect that Queue Notify, ISR Status and
Device Header all exist.

ie. either use legacy mode, or use capabilities.  Never both.

> 
> >     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?

At the moment you give a hybrid model where both are used.  In five
years' time, that's going to be particularly ugly.
> 
> > (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.

But the separation between "legacy" and "modern" will be sharper, making
it easier to excise the legacy portion later.

And in five years' time, people implementing virtio will really thank us
that they can completely ignore the legacy header.

> >    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?

How would we tell?  If we use a new capability struct for it, it's
obvious.  Otherwise, you're going to need to steal more feature bits.

Cheers,
Rusty.
PS.  Sorry, was off sick for a few days.