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. If batching is mostly advisory (with exceptions such as mrg-rxbuf) I don't have any problem with it. Paolo _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
