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

[Michael S. Tsirkin]

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?


> > > >          // 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