virtio: expose added descriptors immediately.

A virtio driver does virtqueue_add_buf() multiple times before finally
calling virtqueue_kick(); previously we only exposed the added buffers
in the virtqueue_kick() call.  This means we don't need a memory
barrier in virtqueue_add_buf(), but it reduces concurrency as the
device (ie. host) can't see the buffers until the kick.

In the unusual (but now possible) case where a driver does add_buf()
and get_buf() without doing a kick, we do need to insert one before
our counter wraps.  Otherwise we could wrap num_added, and later on
not realize that we have passed the marker where we should have
kicked.

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

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 99dc948..36bb6a6 100644 (file)
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -251,9 +251,20 @@ add_head:
 
        /* Put entry in available array (but don't update avail->idx until they
         * do sync). */
-       avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
+       avail = (vq->vring.avail->idx & (vq->vring.num-1));
        vq->vring.avail->ring[avail] = head;
 
+       /* Descriptors and available array need to be set before we expose the
+        * new available array entries. */
+       virtio_wmb(vq);
+       vq->vring.avail->idx++;
+       vq->num_added++;
+
+       /* This is very unlikely, but theoretically possible.  Kick
+        * just in case. */
+       if (unlikely(vq->num_added == (1 << 16) - 1))
+               virtqueue_kick(_vq);
+
        pr_debug("Added buffer head %i to %p\n", head, vq);
        END_USE(vq);
 
@@ -283,13 +294,10 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
         * new available array entries. */
        virtio_wmb(vq);
 
-       old = vq->vring.avail->idx;
-       new = vq->vring.avail->idx = old + vq->num_added;
+       old = vq->vring.avail->idx - vq->num_added;
+       new = vq->vring.avail->idx;
        vq->num_added = 0;
 
-       /* Need to update avail index before checking if we should notify */
-       virtio_mb(vq);
-
        if (vq->event) {
                needs_kick = vring_need_event(vring_avail_event(&vq->vring),
                                              new, old);