Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

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

On Thu, May 30, 2013 at 08:53:45AM -0500, Anthony Liguori wrote:
> Rusty Russell <rusty@rustcorp.com.au> writes:
> 
> > Anthony Liguori <aliguori@us.ibm.com> writes:
> >> Forcing a guest driver change is a really big
> >> deal and I see no reason to do that unless there's a compelling reason
> >> to.
> >>
> >> So we're stuck with the 1.0 config layout for a very long time.
> >
> > We definitely must not force a guest change.  The explicit aim of the
> > standard is that "legacy" and 1.0 be backward compatible.  One
> > deliverable is a document detailing how this is done (effectively a
> > summary of changes between what we have and 1.0).
> 
> If 2.0 is fully backwards compatible, great.  It seems like such a
> difference that that would be impossible but I need to investigate
> further.
> 
> Regards,
> 
> Anthony Liguori

If you look at my patches you'll see how it works.
Basically old guests use BAR0 new ones don't, so
it's easy: BAR0 access means legacy guest.
Only started testing but things seem to work
fine with old guests so far.

I think we need a spec, not just driver code.

Rusty what's the plan? Want me to write it?

> >
> > It's a delicate balancing act.  My plan is to accompany any changes in
> > the standard with a qemu implementation, so we can see how painful those
> > changes are.  And if there are performance implications, measure them.
> >
> > Cheers,
> > Rusty.
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