virtio_ring: expose virtio barriers for use in vringh.

The host side of ring needs this logic too.

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

1 files changed, 57 insertions, 0 deletions

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index 63c6ea199519..ca3ad41c2c82 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -4,6 +4,63 @@
 #include <linux/irqreturn.h>
 #include <uapi/linux/virtio_ring.h>
 
+/*
+ * Barriers in virtio are tricky.  Non-SMP virtio guests can't assume
+ * they're not on an SMP host system, so they need to assume real
+ * barriers.  Non-SMP virtio hosts could skip the barriers, but does
+ * anyone care?
+ *
+ * For virtio_pci on SMP, we don't need to order with respect to MMIO
+ * accesses through relaxed memory I/O windows, so smp_mb() et al are
+ * sufficient.
+ *
+ * For using virtio to talk to real devices (eg. other heterogeneous
+ * CPUs) we do need real barriers.  In theory, we could be using both
+ * kinds of virtio, so it's a runtime decision, and the branch is
+ * actually quite cheap.
+ */
+
+#ifdef CONFIG_SMP
+static inline void virtio_mb(bool weak_barriers)
+{
+        if (weak_barriers)
+                smp_mb();
+        else
+                mb();
+}
+
+static inline void virtio_rmb(bool weak_barriers)
+{
+        if (weak_barriers)
+                smp_rmb();
+        else
+                rmb();
+}
+
+static inline void virtio_wmb(bool weak_barriers)
+{
+        if (weak_barriers)
+                smp_wmb();
+        else
+                wmb();
+}
+#else
+static inline void virtio_mb(bool weak_barriers)
+{
+        mb();
+}
+
+static inline void virtio_rmb(bool weak_barriers)
+{
+        rmb();
+}
+
+static inline void virtio_wmb(bool weak_barriers)
+{
+        wmb();
+}
+#endif
+
 struct virtio_device;
 struct virtqueue;