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

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

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);
>  	vq->split.avail_idx_shadow++;
>  	vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev,
>  						vq->split.avail_idx_shadow);



Something else to try, is to disassemble the code and check the compiler is not broken.

It also might help to replace assigment above with WRITE_ONCE -
it's technically always has been the right thing to do, it's just a big
change (has to be done everywhere if done at all) so we never bothered
and we never hit a compiler that would split or speculate stores ...


> -- 
> 2.44.0