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

[Anthony Liguori]

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?
>   

Features are negotiated.  It lets a host advertise the support of a 
feature and it lets the guest acknowledge it's support of a feature.

Most importantly, why invent a new mechanism when we already have one?

--
Regards,

Anthony Liguori