Re: [PATCH] virtio_ring: Fix the stale index in available ring

[Will Deacon]

On Tue, Mar 26, 2024 at 09:38:55AM +0000, Keir Fraser wrote:
> On Tue, Mar 26, 2024 at 03:49:02AM -0400, Michael S. Tsirkin wrote:
> > > Secondly, the debugging code is enhanced so that the available head for
> > > (last_avail_idx - 1) is read for twice and recorded. It means the available
> > > head for one specific available index is read for twice. I do see the
> > > available heads are different from the consecutive reads. More details
> > > are shared as below.
> > > 
> > > From the guest side
> > > ===================
> > > 
> > > virtio_net virtio0: output.0:id 86 is not a head!
> > > head to be released: 047 062 112
> > > 
> > > avail_idx:
> > > 000  49665
> > > 001  49666  <--
> > >  :
> > > 015  49664
> > 
> > what are these #s 49665 and so on?
> > and how large is the ring?
> > I am guessing 49664 is the index ring size is 16 and
> > 49664 % 16 == 0
> 
> More than that, 49664 % 256 == 0
> 
> So again there seems to be an error in the vicinity of roll-over of
> the idx low byte, as I observed in the earlier log. Surely this is
> more than coincidence?

Yeah, I'd still really like to see the disassembly for both sides of the
protocol here. Gavin, is that something you're able to provide? Worst
case, the host and guest vmlinux objects would be a starting point.

Personally, I'd be fairly surprised if this was a hardware issue.

Will