On Tue, Mar 19, 2024 at 02:59:23PM +1000, Gavin Shan wrote:

On 3/19/24 02:59, Will Deacon wrote:

quoted

On Thu, Mar 14, 2024 at 05:49:23PM +1000, Gavin Shan wrote:

quoted

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 <redacted>
Signed-off-by: Gavin Shan <redacted>
---
  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

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