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Message-ID: <9b3030d1-cb2c-4ce0-8b24-1074b616fc84@redhat.com>
Date: Tue, 19 Mar 2024 16:38:49 +1000
From: Gavin Shan <gshan@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Will Deacon <will@...nel.org>, virtualization@...ts.linux.dev,
linux-kernel@...r.kernel.org, jasowang@...hat.com,
xuanzhuo@...ux.alibaba.com, yihyu@...hat.com, shan.gavin@...il.com,
linux-arm-kernel@...ts.infradead.org,
Catalin Marinas <catalin.marinas@....com>, mochs@...dia.com
Subject: Re: [PATCH] virtio_ring: Fix the stale index in available ring
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
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