[PATCH v3 0/3] vhost: Fix stale available ring entries

[Gavin Shan <gshan@redhat.com>]

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, enable_notify}()
Without smp_rmb(), vq->avail_idx is advanced 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. Besides, the
function vhost_get_avail_idx() is improved to tackle the memory barrier
so that the callers needn't to worry about it.

v2: https://lore.kernel.org/virtualization/46c6a9aa-821c-4013-afe7-61ec05fc9dd4@redhat.com
v1: https://lore.kernel.org/virtualization/66e12633-b2d6-4b9a-9103-bb79770fcafa@redhat.com

Changelog
=========
v3:
  Improved change log                                         (Jason)
  Improved comments and added PATCH[v3 3/3] to execute
  smp_rmb() in vhost_get_avail_idx()                          (Michael)


Gavin Shan (3):
  vhost: Add smp_rmb() in vhost_vq_avail_empty()
  vhost: Add smp_rmb() in vhost_enable_notify()
  vhost: Improve vhost_get_avail_idx() with smp_rmb()

 drivers/vhost/vhost.c | 51 ++++++++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 27 deletions(-)

-- 
2.44.0