Re: [Qemu-devel] [RFC] virtio: Report new guest memory statistics pertinent to memory ballooning (V2)

[Adam Litke]

On Mon, 2009-11-09 at 19:00 +0000, Jamie Lokier wrote:
> Adam Litke wrote:
> > +        s->stats.pswapin = has_feature(dev, VIRTIO_BALLOON_F_RPT_SWAP_OUT) 
> > ?
> > +                                       dev->stats.pswapin : -1;
> 
> (etc.)
> 
> Why not simply have the guest fill in the unused fields with -1, and
> say that's how "no meaningful value" is represented in the ABI?
> 
> All guests have to know about all those fields anyway, for the
> structure layout.  Is there any benefit to specifying feature bits in
> advance over simply storing -1 there?

This holds true for fields that exist but might not be supported by a
guest, but what about the case where we add a new field?  In that case,
qemu and the guest may disagree on the structure layout.

-- 
Thanks,
Adam