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>
diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 595d731..6a1d644 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -292,10 +292,12 @@
/*
* OK, tell virtio_ring.c to set up a virtqueue now we know its size
- * and we've got a pointer to its pages.
+ * and we've got a pointer to its pages. Note that we set weak_barriers
+ * to 'true': the host just a(nother) SMP CPU, so we only need inter-cpu
+ * barriers.
*/
- vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN,
- vdev, lvq->pages, lg_notify, callback, name);
+ vq = vring_new_virtqueue(lvq->config.num, LGUEST_VRING_ALIGN, vdev,
+ true, lvq->pages, lg_notify, callback, name);
if (!vq) {
err = -ENOMEM;
goto unmap;