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

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

On Thu, Mar 14, 2024 at 08:15:22PM +1000, Gavin Shan wrote:
> On 3/14/24 18:05, Michael S. Tsirkin 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@vger.kernel.org
> > > Reported-by: Yihuang Yu <yihyu@redhat.com>
> > > Signed-off-by: Gavin Shan <gshan@redhat.com>
> > > ---
> > >   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);
> > 
> > 
> > I don't get what is going on here. Any explanation why virtio_wmb is not
> > enough besides "it does not work"?
> > 
> 
> The change is replacing instruction "dmb" with "dsb". "dsb" is stronger barrier
> than "dmb" because "dsb" ensures that all memory accesses raised before this
> instruction is completed when the 'dsb' instruction completes. However, "dmb"
> doesn't guarantee the order of completion of the memory accesses.
>
> So 'vq->split.vring.avail->idx = cpu_to_virtio(_vq->vdev, vq->split.avail_idx_shadow)'
> can be completed before 'vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head)'.

Completed as observed by which CPU?
We have 2 writes that we want observed by another CPU in order.
So if CPU observes a new value of idx we want it to see
new value in ring.
This is standard use of smp_wmb()
How are these 2 writes different?

What DMB does, is that is seems to ensure that effects
of 'vq->split.vring.avail->idx = cpu_to_virtio(_vq->vdev, vq->split.avail_idx_shadow)'
are observed after effects of
'vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head)'.




> The stronger barrier 'dsb' ensures the completion order as we expected.
> 
>     virtio_wmb(true)         virt_mb(false)
>       virt_wmb                 mb
>         __smp_wmb               __mb
>           dmb(ishst)              dsb(sy)

First, why would you want a non smp barrier when you are
synchronizing with another CPU? This is what smp_ means ...


> Extraced from ARMv9 specificaton
> ================================
> The DMB instruction is a memory barrier instruction that ensures the relative
> order of memory accesses before the barrier with memory accesses after the
> barrier. The DMB instruction _does not_ ensure the completion of any of the
> memory accesses for which it ensures relative order.

Isn't this exactly what we need?

> A DSB instruction is a memory barrier that ensures that memory accesses that
> occur before the DSB instruction have __completed__ before the completion of
> the DSB instruction.

This seems appropriate for when you want to order things more
strongly. I do not get why it's necessary here.

> In doing this, it acts as a stronger barrier than a DMB
> and all ordering that is created by a DMB with specific options is also generated
> by a DSB with the same options.
> 
> > >   	vq->split.avail_idx_shadow++;
> > >   	vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
> > >   						vq->split.avail_idx_shadow);
> > > -- 
> > > 2.44.0
> > 
> 
> Thanks,
> Gavin