tree 0eb215813f3066bf634a395d02cf4aab3ba94fc6
parent 97a545ab6ce922a0f868d192718a48a0091ebc5e
author Michael S. Tsirkin <mst@redhat.com> 1264632143 +0200
committer Rusty Russell <rusty@rustcorp.com.au> 1266983545 +1030

virtio: use smp_XX barriers on SMP

virtio is communicating with a virtual "device" that actually runs on
another host processor. Thus SMP barriers can be used to control
memory access ordering.

Where possible, we should use SMP barriers which are more lightweight than
mandatory barriers, because mandatory barriers also control MMIO effects on
accesses through relaxed memory I/O windows (which virtio does not use)
(compare specifically smp_rmb and rmb on x86_64).

We can't just use smp_mb and friends though, because
we must force memory ordering even if guest is UP since host could be
running on another CPU, but SMP barriers are defined to barrier() in
that configuration. So, for UP fall back to mandatory barriers instead.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>