virtio: harsher barriers for rpmsg.

We were cheating with our barriers; using the smp ones rather than the
real device ones.  That was fine, until rpmsg came along, which is
used to talk to a real device (a non-SMP CPU).

Unfortunately, just putting back the real barriers (reverting
d57ed95d) causes a performance regression on virtio-pci.  In
particular, Amos reports netbench's TCP_RR over virtio_net CPU
utilization increased up to 35% while throughput went down by up to
14%.

By comparison, this branch is in the noise.

Reference: https://lkml.org/lkml/2011/12/11/22

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

1 files changed, 21 insertions, 13 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c7a2c208f6e..50da9204609 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -28,17 +28,20 @@
 #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()
+ * 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_rmb(); else rmb(); } 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() mb()
-#define virtio_rmb() rmb()
-#define virtio_wmb() wmb()
+#define virtio_mb(vq) mb()
+#define virtio_rmb(vq) rmb()
+#define virtio_wmb(vq) wmb()
 #endif
 
 #ifdef DEBUG
@@ -77,6 +80,9 @@ struct vring_virtqueue
 	/* Actual memory layout for this queue */
 	struct vring vring;
 
+	/* Can we use weak barriers? */
+	bool weak_barriers;
+
 	/* Other side has made a mess, don't try any more. */
 	bool broken;
 
@@ -245,14 +251,14 @@ void virtqueue_kick(struct virtqueue *_vq)
 	START_USE(vq);
 	/* Descriptors and available array need to be set before we expose the
 	 * new available array entries. */
-	virtio_wmb();
+	virtio_wmb(vq);
 
 	old = vq->vring.avail->idx;
 	new = vq->vring.avail->idx = old + vq->num_added;
 	vq->num_added = 0;
 
 	/* Need to update avail index before checking if we should notify */
-	virtio_mb();
+	virtio_mb(vq);
 
 	if (vq->event ?
 	    vring_need_event(vring_avail_event(&vq->vring), new, old) :
@@ -314,7 +320,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
 	}
 
 	/* Only get used array entries after they have been exposed by host. */
-	virtio_rmb();
+	virtio_rmb(vq);
 
 	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;
@@ -337,7 +343,7 @@ 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();
+		virtio_mb(vq);
 	}
 
 	END_USE(vq);
@@ -366,7 +372,7 @@ 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();
+	virtio_mb(vq);
 	if (unlikely(more_used(vq))) {
 		END_USE(vq);
 		return false;
@@ -393,7 +399,7 @@ 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();
+	virtio_mb(vq);
 	if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
 		END_USE(vq);
 		return false;
@@ -453,6 +459,7 @@ EXPORT_SYMBOL_GPL(vring_interrupt);
 struct virtqueue *vring_new_virtqueue(unsigned int num,
 				      unsigned int vring_align,
 				      struct virtio_device *vdev,
+				      bool weak_barriers,
 				      void *pages,
 				      void (*notify)(struct virtqueue *),
 				      void (*callback)(struct virtqueue *),
@@ -476,6 +483,7 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
 	vq->vq.vdev = vdev;
 	vq->vq.name = name;
 	vq->notify = notify;
+	vq->weak_barriers = weak_barriers;
 	vq->broken = false;
 	vq->last_used_idx = 0;
 	vq->num_added = 0;