1 files changed, 22 insertions, 4 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1ee97d402a48..827f7e042610 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -21,6 +21,24 @@
 #include <linux/virtio_config.h>
 #include <linux/device.h>
 
+/* virtio guest is communicating with a virtual "device" that actually runs on
+ * a host processor.  Memory barriers are used to control SMP effects. */
+#ifdef CONFIG_SMP
+/* Where possible, use SMP barriers which are more lightweight than mandatory
+ * barriers, because mandatory barriers control MMIO effects on accesses
+ * through relaxed memory I/O windows (which virtio does not use). */
+#define virtio_mb() smp_mb()
+#define virtio_rmb() smp_rmb()
+#define virtio_wmb() smp_wmb()
+#else
+/* 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 fall back to mandatory barriers instead. */
+#define virtio_mb() mb()
+#define virtio_rmb() rmb()
+#define virtio_wmb() wmb()
+#endif
+
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
 #define BAD_RING(_vq, fmt, args...)                             \
@@ -220,13 +238,13 @@ static void vring_kick(struct virtqueue *_vq)
         START_USE(vq);
         /* Descriptors and available array need to be set before we expose the
          * new available array entries. */
-        wmb();
+        virtio_wmb();
 
         vq->vring.avail->idx += vq->num_added;
         vq->num_added = 0;
 
         /* Need to update avail index before checking if we should notify */
-        mb();
+        virtio_mb();
 
         if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))
                 /* Prod other side to tell it about changes. */
@@ -285,7 +303,7 @@ static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
         }
 
         /* Only get used array entries after they have been exposed by host. */
-        rmb();
+        virtio_rmb();
 
         i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
         *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;
@@ -323,7 +341,7 @@ static bool vring_enable_cb(struct virtqueue *_vq)
         /* We optimistically turn back on interrupts, then check if there was
          * more to do. */
         vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
-        mb();
+        virtio_mb();
         if (unlikely(more_used(vq))) {
                 END_USE(vq);
                 return false;