On Fri, Feb 10, 2017 at 12:32:49PM +0100, Paolo Bonzini wrote:
>
>
> 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.
Well what I meant is that with qsize not a power of 2 you can still do
this but have to do everything mod N*qsize as opposed to mod 2^16. So
you need a branch there - easiest to do if you do signed math.
int nheads = avail - last_avail;
/*Check and handle index wrap-around */
if (unlikely(nheads < 0)) {
nheads += N_qsize;
}
if (nheads < 0 || nheads > vdev->vq[i].vring.num) {
error_report(...);
return -1;
}
This can only catch bugs if N > 1
> 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
Same thing here, this never triggers if vring.num == 2^16
--
MST
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