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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
