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>

drivers/virtio/virtio_ring.c

@@ -24,27 +24,6 @@
 #include <linux/module.h>
 #include <linux/hrtimer.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-pci does not use). */
-#define virtio_mb(vq) \
-	do { if ((vq)->weak_barriers) smp_mb(); else mb(); } while(0)
-#define virtio_rmb(vq) \
-	do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0)
-#define virtio_wmb(vq) \
-	do { if ((vq)->weak_barriers) smp_wmb(); else wmb(); } while(0)
-#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(vq) mb()
-#define virtio_rmb(vq) rmb()
-#define virtio_wmb(vq) wmb()
-#endif
-
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
 #define BAD_RING(_vq, fmt, args...)				\
@@ -276,7 +255,7 @@ int virtqueue_add_buf(struct virtqueue *_vq,
 
 	/* Descriptors and available array need to be set before we expose the
 	 * new available array entries. */
-	virtio_wmb(vq);
+	virtio_wmb(vq->weak_barriers);
 	vq->vring.avail->idx++;
 	vq->num_added++;
 
@@ -312,7 +291,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	START_USE(vq);
 	/* We need to expose available array entries before checking avail
 	 * event. */
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 
 	old = vq->vring.avail->idx - vq->num_added;
 	new = vq->vring.avail->idx;
@@ -436,7 +415,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 	}
 
 	/* Only get used array entries after they have been exposed by host. */
-	virtio_rmb(vq);
+	virtio_rmb(vq->weak_barriers);
 
 	last_used = (vq->last_used_idx & (vq->vring.num - 1));
 	i = vq->vring.used->ring[last_used].id;
@@ -460,7 +439,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 	 * the read in the next get_buf call. */
 	if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
 		vring_used_event(&vq->vring) = vq->last_used_idx;
-		virtio_mb(vq);
+		virtio_mb(vq->weak_barriers);
 	}
 
 #ifdef DEBUG
@@ -513,7 +492,7 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 	 * entry. Always do both to keep code simple. */
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
 	vring_used_event(&vq->vring) = vq->last_used_idx;
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 	if (unlikely(more_used(vq))) {
 		END_USE(vq);
 		return false;
@@ -553,7 +532,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 	/* TODO: tune this threshold */
 	bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4;
 	vring_used_event(&vq->vring) = vq->last_used_idx + bufs;
-	virtio_mb(vq);
+	virtio_mb(vq->weak_barriers);
 	if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
 		END_USE(vq);
 		return false;

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;