On Sun, Nov 13, 2011 at 11:03:13PM +0200, Michael S. Tsirkin wrote:

On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:

quoted

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.

Signed-off-by: Rusty Russell <redacted>

In the past I played with a patch like this, but I didn't see a
performance gain either way. Do you see any gain?

I'm a bit concerned that with this patch, a buggy driver that
adds more than 2^16 descriptors without a kick
would seem to work sometimes. Let's add WARN_ON(vq->num_added > (1 << 16))?

Thinking about it more - it might be tricky for drivers
to ensure this. add used to fail when vq is full, but now
driver might do get between add and notify:
	lock
	add_buf * N
	prep
	unlock
	lock
	get_buf * N
	unlock
	lock
	add_buf
	prep
	unlock
	notify

and since add was followed by get, this doesn't fail.

So the right thing to do I think is to either ignore indexes and assume
a kick is needed, something like:
if vq->num_added >= (1 << 15))
	needs_kick = true
(note: maybe it's 1<<16, and maybe >, but 1<<15 is plenty anyway)

Or alternatively, fail add when num_added is too large.

quoted

---
 drivers/virtio/virtio_ring.c |   37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -227,9 +227,15 @@ 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->vring.avail->idx++;
+	vq->num_added++;
+
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
 

@@ -248,13 +254,10 @@ bool virtqueue_kick_prepare(struct virtq
 	 * new available array entries. */
 	virtio_wmb();
 
-	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();
-
 	if (vq->event) {
 		needs_kick = vring_need_event(vring_avail_event(&vq->vring),
 					      new, old);

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