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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 19, 2024 at 04:38:49PM +1000, Gavin Shan wrote:
> 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);

So we see that just changing the timing masks the race.
What are you using on the host side? vhost or qemu?

> 
> > 
> > >          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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