The issue was reported by Yihuang Yu on NVidia's grace-hopper (ARM64)
platform. The wrong head (available ring entry) is seen by the guest
when running 'netperf' on the guest and running 'netserver' on another
NVidia's grace-grace machine.

  /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64      \
  -accel kvm -machine virt,gic-version=host -cpu host          \
  -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
  -m 4096M,slots=16,maxmem=64G                                 \
  -object memory-backend-ram,id=mem0,size=4096M                \
   :                                                           \
  -netdev tap,id=tap0,vhost=true                               \
  -device virtio-net-pci,bus=pcie.8,netdev=tap0,mac=52:54:00:f1:26:b0
   :
  guest# ifconfig eth0 | grep 'inet addr'
  inet addr:10.26.1.220
  guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
  virtio_net virtio0: output.0:id 100 is not a head!

There is missed smp_rmb() in vhost_vq_avail_empty() and vhost_enable_notify().
Without smp_rmb(), vq->avail_idx is increased but the available ring
entries aren't arriving to vhost side yet. So a stale available ring
entry can be fetched in vhost_get_vq_desc().

Fix it by adding smp_rmb() in those two functions. Note that I need
two patches so that they can be easily picked up by the stable kernel.
With the changes, I'm unable to hit the issue again.

Gavin Shan (2):
  vhost: Add smp_rmb() in vhost_vq_avail_empty()
  vhost: Add smp_rmb() in vhost_enable_notify()

 drivers/vhost/vhost.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

-- 
2.44.0

On 3/27/24 09:38, Gavin Shan wrote:
> The issue was reported by Yihuang Yu on NVidia's grace-hopper (ARM64)
> platform. The wrong head (available ring entry) is seen by the guest
> when running 'netperf' on the guest and running 'netserver' on another
> NVidia's grace-grace machine.
> 
>    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64      \
>    -accel kvm -machine virt,gic-version=host -cpu host          \
>    -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
>    -m 4096M,slots=16,maxmem=64G                                 \
>    -object memory-backend-ram,id=mem0,size=4096M                \
>     :                                                           \
>    -netdev tap,id=tap0,vhost=true                               \
>    -device virtio-net-pci,bus=pcie.8,netdev=tap0,mac=52:54:00:f1:26:b0
>     :
>    guest# ifconfig eth0 | grep 'inet addr'
>    inet addr:10.26.1.220
>    guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
>    virtio_net virtio0: output.0:id 100 is not a head!
> 
> There is missed smp_rmb() in vhost_vq_avail_empty() and vhost_enable_notify().
> Without smp_rmb(), vq->avail_idx is increased but the available ring
> entries aren't arriving to vhost side yet. So a stale available ring
> entry can be fetched in vhost_get_vq_desc().
> 
> Fix it by adding smp_rmb() in those two functions. Note that I need
> two patches so that they can be easily picked up by the stable kernel.
> With the changes, I'm unable to hit the issue again.
> 
> Gavin Shan (2):
>    vhost: Add smp_rmb() in vhost_vq_avail_empty()
>    vhost: Add smp_rmb() in vhost_enable_notify()
> 
>   drivers/vhost/vhost.c | 22 ++++++++++++++++++++--
>   1 file changed, 20 insertions(+), 2 deletions(-)
> 

Sorry, I was supposed to copy Will. Amending for it.

Thanks,
Gavin