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Subject: Re: [virtio-dev] RFC: Doorbell suppression, packed-ring mode and hardware offload

On Fri, Feb 15, 2019 at 11:59:55AM +0800, Jason Wang wrote:
> On 2019/2/14 äå11:41, Michael S. Tsirkin wrote:
> > > I think it's as simple as increase the avail idx by X? Since descriptor were
> > > used in order, device can just read the next X-1 descriptors in this case.
> > > 
> > > Thanks
> > Right so a spec change would be needed, it's not transparent to guest.
> With the change, IN_ORDER + split_ring becomes something like submission
> queue (descriptor ring) + completion queue (used ring). And used ring access
> could be eliminated sometime, e.g for net for TX, we don't care about used
> len.

Oh yes but fundamentally this becomes very close to packed ring. So I'm
not sure yet another option is justified by a small gain in PPS,
especially considering that it depends on in order and so doesn't
support zero copy.

> What's more interesting is, this avail idx optimization is not required for
> out of order completion which means it could be used for e.g block or SCSI
> device.

I don't get the last sentence. This only works for in order right?


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