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

[Will Deacon]

On Tue, Mar 19, 2024 at 02:59:23PM +1000, Gavin Shan wrote:
> On 3/19/24 02:59, Will Deacon wrote:
> > On Thu, Mar 14, 2024 at 05:49:23PM +1000, Gavin Shan wrote:
> > > The issue is reported by Yihuang Yu who have 'netperf' test on
> > > NVidia's grace-grace and grace-hopper machines. The 'netperf'
> > > client is started in the VM hosted by grace-hopper machine,
> > > while the 'netperf' server is running on grace-grace machine.
> > > 
> > > The VM is started with virtio-net and vhost has been enabled.
> > > We observe a error message spew from VM and then soft-lockup
> > > report. The error message indicates the data associated with
> > > the descriptor (index: 135) has been released, and the queue
> > > is marked as broken. It eventually leads to the endless effort
> > > to fetch free buffer (skb) in drivers/net/virtio_net.c::start_xmit()
> > > and soft-lockup. The stale index 135 is fetched from the available
> > > ring and published to the used ring by vhost, meaning we have
> > > disordred write to the available ring element and available index.
> > > 
> > >    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64              \
> > >    -accel kvm -machine virt,gic-version=host                            \
> > >       :                                                                 \
> > >    -netdev tap,id=vnet0,vhost=on                                        \
> > >    -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 \
> > > 
> > >    [   19.993158] virtio_net virtio1: output.0:id 135 is not a head!
> > > 
> > > Fix the issue by replacing virtio_wmb(vq->weak_barriers) with stronger
> > > virtio_mb(false), equivalent to replaced 'dmb' by 'dsb' instruction on
> > > ARM64. It should work for other architectures, but performance loss is
> > > expected.
> > > 
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Reported-by: Yihuang Yu <yihyu@xxxxxxxxxx>
> > > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > > ---
> > >   drivers/virtio/virtio_ring.c | 12 +++++++++---
> > >   1 file changed, 9 insertions(+), 3 deletions(-)
> > > 
> > > 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();

It's pretty weird that __dma_mb() is "broken" but __smp_mb() "works". How
confident are you in that result?

Will