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

[Michael S. Tsirkin]

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?

> 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

> 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