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

[Gavin Shan <gshan@redhat.com>]

On 3/20/24 04:22, Will Deacon wrote:
> On Tue, Mar 19, 2024 at 02:59:23PM +1000, Gavin Shan wrote:
>> On 3/19/24 02:59, Will Deacon wrote:
>>>>    drivers/virtio/virtio_ring.c | 12 +++++++++---
>>>>    1 file changed, 9 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>>> index 49299b1f9ec7..7d852811c912 100644
>>>> --- a/drivers/virtio/virtio_ring.c
>>>> +++ b/drivers/virtio/virtio_ring.c
>>>> @@ -687,9 +687,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>>>>    	avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
>>>>    	vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
>>>> -	/* Descriptors and available array need to be set before we expose the
>>>> -	 * new available array entries. */
>>>> -	virtio_wmb(vq->weak_barriers);
>>>> +	/*
>>>> +	 * Descriptors and available array need to be set before we expose
>>>> +	 * the new available array entries. virtio_wmb() should be enough
>>>> +	 * to ensuere the order theoretically. However, a stronger barrier
>>>> +	 * is needed by ARM64. Otherwise, the stale data can be observed
>>>> +	 * by the host (vhost). A stronger barrier should work for other
>>>> +	 * architectures, but performance loss is expected.
>>>> +	 */
>>>> +	virtio_mb(false);
>>>>    	vq->split.avail_idx_shadow++;
>>>>    	vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
>>>>    						vq->split.avail_idx_shadow);
>>>
>>> Replacing a DMB with a DSB is _very_ unlikely to be the correct solution
>>> here, especially when ordering accesses to coherent memory.
>>>
>>> In practice, either the larger timing different from the DSB or the fact
>>> that you're going from a Store->Store barrier to a full barrier is what
>>> makes things "work" for you. Have you tried, for example, a DMB SY
>>> (e.g. via __smb_mb()).
>>>
>>> We definitely shouldn't take changes like this without a proper
>>> explanation of what is going on.
>>>
>>
>> Thanks for your comments, Will.
>>
>> Yes, DMB should work for us. However, it seems this instruction has issues on
>> NVidia's grace-hopper. It's hard for me to understand how DMB and DSB works
>> from hardware level. I agree it's not the solution to replace DMB with DSB
>> before we fully understand the root cause.
>>
>> I tried the possible replacement like below. __smp_mb() can avoid the issue like
>> __mb() does. __ndelay(10) can avoid the issue, but __ndelay(9) doesn't.
>>
>> 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);
>>
>>          /* Descriptors and available array need to be set before we expose the
>>           * new available array entries. */
>>          // Broken: virtio_wmb(vq->weak_barriers);
>>          // Broken: __dma_mb();
>>          // Work:   __mb();
>>          // Work:   __smp_mb();
> 
> It's pretty weird that __dma_mb() is "broken" but __smp_mb() "works". How
> confident are you in that result?
> 

Yes, __dma_mb() is even stronger than __smp_mb(). I retried the test, showing
that both __dma_mb() and __smp_mb() work for us. I had too many tests yesterday
and something may have been messed up.

Instruction         Hitting times in 10 tests
---------------------------------------------
__smp_wmb()         8
__smp_mb()          0
__dma_wmb()         7
__dma_mb()          0
__mb()              0
__wmb()             0

It's strange that __smp_mb() works, but __smp_wmb() fails. It seems we need a
read barrier here. I will try WRITE_ONCE() + __smp_wmb() as suggested by Michael
in another reply. Will update the result soon.

Thanks,
Gavin