virtio: harsher barriers for rpmsg.

We were cheating with our barriers; using the smp ones rather than the
real device ones.  That was fine, until rpmsg came along, which is
used to talk to a real device (a non-SMP CPU).

Unfortunately, just putting back the real barriers (reverting
d57ed95d) causes a performance regression on virtio-pci.  In
particular, Amos reports netbench's TCP_RR over virtio_net CPU
utilization increased up to 35% while throughput went down by up to
14%.

By comparison, this branch is in the noise.

Reference: https://lkml.org/lkml/2011/12/11/22

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 36be0f6e18a9..e338730c2660 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -168,6 +168,7 @@ struct virtqueue;
 struct virtqueue *vring_new_virtqueue(unsigned int num,
                                       unsigned int vring_align,
                                       struct virtio_device *vdev,
+                                      bool weak_barriers,
                                       void *pages,
                                       void (*notify)(struct virtqueue *vq),
                                       void (*callback)(struct virtqueue *vq),