Re: [virtio-dev] packed ring layout proposal

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/09/2016 21:14, Michael S. Tsirkin wrote:
> > But I prefer all these tricks to be hidden within the driver.  It seems
> > a good idea in the beginning to rig the device to second-guess what a
> > driver could do, but then it makes things awkward.  (This is also why
> > I'd rather get rid of VRING_DESC_F_NEXT).
> 
> Right, I'll CC dpdk list on this proposal. As dpdk uses _NEXT almost
> exclusively I'd like to make sure we are not messing things up.
> 
> Maybe the right thing to do is to disallow _NEXT if _INDIRECT is
> negotiated. Each device can then decide whether it wants to
> use _INDIRECT or _NEXT (or both). How's that?

Still a bit of feature creep if we can avoid it, but at least it lets
you write two fast loops to parse the descriptors.  So that's already a
huge improvement.

Negotiating _INDIRECT would still allow a single direct buffer.

Just one thing: in the  _NEXT case, does the driver write only one
available descriptor for the head (effectively ignoring desc.index on
all descriptors but the first)?  Or does it have to write all the
descriptors?  If the latter, _INDIRECT would almost surely end up faster.

Paolo