From ee7cd8981e15bcb365fc762afe3fc47b8242f630 Mon Sep 17 00:00:00 2001
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 12 Jan 2012 15:44:43 +1030
Subject: [PATCH] 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>
---
drivers/virtio/virtio_ring.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 99dc9480f3fe5..36bb6a6137283 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -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);
--
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