kvm-all.c: define smp_wmb and use it for coalesced mmio

Acked-by: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/qemu-barrier.h b/qemu-barrier.h
new file mode 100644
index 0000000..3bd1075
--- /dev/null
+++ b/qemu-barrier.h
@@ -0,0 +1,7 @@
+#ifndef __QEMU_BARRIER_H
+#define __QEMU_BARRIER_H 1
+
+/* FIXME: arch dependant, x86 version */
+#define smp_wmb()   asm volatile("" ::: "memory")
+
+#endif