On Mon, 14 Nov 2011 08:56:06 +0200, "Michael S. Tsirkin" [off-list ref] wrote:

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

quoted

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.

Right, the driver could, in theory, do:
        add_buf()
        if (!get_buf())
                notify()

But we don't allow that at the moment in our API: we insist on a notify
occasionally.  Noone does this at the moment, so a WARN_ON is correct.

If you're just add_buf() without the get_buf() then add_buf() will fail
already.

Here's my current variant:

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

@@ -245,9 +245,19 @@ 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++;
+
+	/* If you haven't kicked in this long, you're probably doing something
+	 * wrong. */
+	WARN_ON(vq->num_added > vq->vring.num);
+
 	pr_debug("Added buffer head %i to %p\n", head, vq);
 	END_USE(vq);
 

It's hard to write a useful WARN_ON() for the "you should kick more
regularly" case (we could take timestamps if DEBUG is defined, I guess),
so let's leave this until someone actually trips it.

Thanks,
Rusty.