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

[Paolo Bonzini <pbonzini@redhat.com>]

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