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

[Gavin Shan]

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.

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.

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

avail_head:
000  062
001  047  <--
  :
015  112

 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