Re: [virtio-dev] packed ring layout proposal v2

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

On Thu, Feb 09, 2017 at 04:48:53PM +0100, Paolo Bonzini wrote:
> 
> 
> On 08/02/2017 20:59, Michael S. Tsirkin wrote:
> > We couldn't decide what's better for everyone in 1.0 days and I doubt
> > we'll be able to now, but yes, benchmarking is needed to make
> > sire it's required. Very easy to remove or not to use/support in
> > drivers/devices though.
> 
> Fair enough, but of course then we must specify that devices MUST
> support either VRING_DESC_F_NEXT or VRING_DESC_F_INDIRECT, and drivers
> SHOULD support both (or use neither).
> 
> The drivers' part adds to the implementation burden, which is why I
> wanted to remove it.  Alternatively we can say that indirect is
> mandatory for both devices and drivers (and save a feature bit), while
> VRING_DESC_F_NEXT is optional.
> 
> >>> * Non power-of-2 ring sizes
> >>>
> >>> As the ring simply wraps around, there's no reason to
> >>> require ring size to be power of two.
> >>> It can be made a separate feature though.
> >>
> >> Power of 2 ring sizes are required in order to ignore the high bits of
> >> the indices.  With non-power-of-2 sizes you are forced to keep the
> >> indices less than the ring size.
> > 
> > Right. So
> > 
> > 	if (unlikely(idx++ > size))
> > 		idx = 0;
> > 
> > OTOH ring size that's twice larger than necessary
> > because of power of two requirements wastes cache.
> 
> I don't know.  Power of 2 ring size is pretty standard, I'd rather avoid
> the complication and the gratuitous difference with 1.0.

I thought originally there's a reason 1.0 rings had to be powers of two
but now I don't see why. OK, we can make it a feature flag later if we
want to.

> If batching is mostly advisory (with exceptions such as mrg-rxbuf) I
> don't have any problem with it.
> 
> Paolo