Commit ee7cd898 authored by Rusty Russell's avatar Rusty Russell

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: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 3b720b8c
...@@ -251,9 +251,20 @@ int virtqueue_add_buf(struct virtqueue *_vq, ...@@ -251,9 +251,20 @@ int virtqueue_add_buf(struct virtqueue *_vq,
/* Put entry in available array (but don't update avail->idx until they /* Put entry in available array (but don't update avail->idx until they
* do sync). */ * 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; 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); pr_debug("Added buffer head %i to %p\n", head, vq);
END_USE(vq); END_USE(vq);
...@@ -283,13 +294,10 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq) ...@@ -283,13 +294,10 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
* new available array entries. */ * new available array entries. */
virtio_wmb(vq); virtio_wmb(vq);
old = vq->vring.avail->idx; old = vq->vring.avail->idx - vq->num_added;
new = vq->vring.avail->idx = old + vq->num_added; new = vq->vring.avail->idx;
vq->num_added = 0; vq->num_added = 0;
/* Need to update avail index before checking if we should notify */
virtio_mb(vq);
if (vq->event) { if (vq->event) {
needs_kick = vring_need_event(vring_avail_event(&vq->vring), needs_kick = vring_need_event(vring_avail_event(&vq->vring),
new, old); new, old);
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment