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

[Gavin Shan <gshan@redhat.com>]

On 3/19/24 17:04, Michael S. Tsirkin wrote:
> On Tue, Mar 19, 2024 at 04:54:15PM +1000, Gavin Shan wrote:
>> On 3/19/24 16:10, Michael S. Tsirkin wrote:
>>> On Tue, Mar 19, 2024 at 02:09:34AM -0400, Michael S. Tsirkin wrote:
>>>> On Tue, Mar 19, 2024 at 02:59:23PM +1000, Gavin Shan wrote:
>>>>> On 3/19/24 02:59, Will Deacon 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();
>>>
>>> Did you try __smp_wmb ? And wmb?
>>>
>>
>> virtio_wmb(false) is equivalent to __smb_wmb(), which is broken.
>>
>> __wmb() works either. No issue found with it.
> 
> Oh interesting. So how do smp_mb() and wmb() disassemble on this
> platform? Can you please check?
> 

I don't see they have been translated wrongly on Nvidia's grace-hopper:

===> virtio_wmb(vq->weak_barriers)

0xffff8000807b07c8 <+1168>:  ldrb    w0, [x20, #66]
0xffff8000807b07cc <+1172>:  cbz     w0, 0xffff8000807b089c <virtqueue_add_split+1380>
0xffff8000807b07d0 <+1176>:  dmb     ishst     // same to __smp_wmb()
     :
0xffff8000807b089c <+1380>:  dmb     oshst     // same to __dma_wmb()
0xffff8000807b08a0 <+1384>:  b       0xffff8000807b07d4 <virtqueue_add_split+1180>

===> wmb()

0xffff8000807b07c8 <+1168>:  dsb     st


> 
>>>>>           // 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?
>>>
>>> And keep virtio_wmb above?
>>>
>>
>> The result has been shared through a separate reply.
>>
>>>>
>>>>>           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