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virtio: expose added descriptors immediately.

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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>
This commit is contained in:
Rusty Russell 2012-01-12 15:44:43 +10:30
parent 3b720b8c86
commit ee7cd8981e
1 changed files with 14 additions and 6 deletions
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20
drivers/virtio/virtio_ring.c
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@ -251,9 +251,20 @@ int virtqueue_add_buf(struct virtqueue *_vq,
/* 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);