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

[Keir Fraser]

On Tue, Mar 26, 2024 at 03:49:02AM -0400, Michael S. Tsirkin wrote:
> On Mon, Mar 25, 2024 at 05:34:29PM +1000, Gavin Shan wrote:
> > 
> > On 3/20/24 17:14, Michael S. Tsirkin wrote:
> > > On Wed, Mar 20, 2024 at 03:24:16PM +1000, Gavin Shan wrote:
> > > > On 3/20/24 10:49, Michael S. Tsirkin wrote:>
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index 6f7e5010a673..79456706d0bd 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -685,7 +685,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> > > > >    	/* Put entry in available array (but don't update avail->idx until they
> > > > >    	 * do sync). */
> > > > >    	avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
> > > > > -	vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
> > > > > +	u16 headwithflag = head | (q->split.avail_idx_shadow & ~(vq->split.vring.num - 1));
> > > > > +	vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, headwithflag);
> > > > >    	/* Descriptors and available array need to be set before we expose the
> > > > >    	 * new available array entries. */
> > > > > 
> > 
> > Ok, Michael. I continued with my debugging code. It still looks like a
> > hardware bug on NVidia's grace-hopper. I really think NVidia needs to be
> > involved for the discussion, as suggested by you.
> 
> Do you have a support contact at Nvidia to report this?
> 
> > Firstly, I bind the vhost process and vCPU thread to CPU#71 and CPU#70.
> > Note that I have only one vCPU in my configuration.
> 
> Interesting but is guest built with CONFIG_SMP set?

arm64 is always built CONFIG_SMP.

> > 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?

 -- Keir

> > avail_head:
> 
> 
> is this the avail ring contents?
> 
> > 000  062
> > 001  047  <--
> >  :
> > 015  112
> 
> 
> What are these arrows pointing at, btw?
> 
> 
> > From the host side
> > ==================
> > 
> > avail_idx
> > 000  49663
> > 001  49666  <---
> >  :
> > 
> > avail_head
> > 000  062  (062)
> > 001  047  (047)  <---
> >  :
> > 015  086  (112)          // head 086 is returned from the first read,
> >                          // but head 112 is returned from the second read
> > 
> > vhost_get_vq_desc: Inconsistent head in two read (86 -> 112) for avail_idx 49664
> > 
> > Thanks,
> > Gavin
> 
> OK thanks so this proves it is actually the avail ring value.
> 
> -- 
> MST
>