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

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/02/2017 19:24, Michael S. Tsirkin wrote:
>> 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.

The reason is that it allows indices to be free running.  This is an 
example of QEMU code that requires that:

            nheads = vring_avail_idx(&vdev->vq[i]) - vdev->vq[i].last_avail_idx;
            /* Check it isn't doing strange things with descriptor numbers. */
            if (nheads > vdev->vq[i].vring.num) {
                error_report("VQ %d size 0x%x Guest index 0x%x "
                             "inconsistent with Host index 0x%x: delta 0x%x",
                             i, vdev->vq[i].vring.num,
                             vring_avail_idx(&vdev->vq[i]),
                             vdev->vq[i].last_avail_idx, nheads);
                return -1;
            }

Paolo