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

[Gavin Shan <gshan@redhat.com>]

On 3/19/24 16:09, Michael S. Tsirkin wrote:

>>>> 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();
>>          // Work:   __ndelay(100);
>>          // Work:   __ndelay(10);
>>          // Broken: __ndelay(9);
>>
>>         vq->split.avail_idx_shadow++;
>>          vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
>>                                                  vq->split.avail_idx_shadow);
> 
> What if you stick __ndelay here?
> 

        /* 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. */
         virtio_wmb(vq->weak_barriers);
         vq->split.avail_idx_shadow++;
         vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
                                                 vq->split.avail_idx_shadow);
         /* Try __ndelay(x) here as Michael suggested
          *
          * Work:      __ndelay(200);    possiblly make it hard to reproduce
          * Broken:    __ndelay(100);
          * Broken:    __ndelay(20);
          * Broken:    __ndelay(10);
          */
         __ndelay(200);


> 
>>          vq->num_added++;
>>
>>          pr_debug("Added buffer head %i to %p\n", head, vq);
>>          END_USE(vq);
>>          :
>> }
>>
>> I also tried to measure the consumed time for various barrier-relative instructions using
>> ktime_get_ns() which should have consumed most of the time. __smb_mb() is slower than
>> __smp_wmb() but faster than __mb()
>>
>>      Instruction           Range of used time in ns
>>      ----------------------------------------------
>>      __smp_wmb()           [32  1128032]
>>      __smp_mb()            [32  1160096]
>>      __mb()                [32  1162496]
>>

Thanks,
Gavin