On Mon, 21 Nov 2011 13:57:04 +0200, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

On Mon, Nov 21, 2011 at 12:18:45PM +1030, Rusty Russell wrote:

quoted

On Wed, 16 Nov 2011 09:18:38 +0200, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

My unlocked kick patches will trip this warning: they make
virtio-net do add + get without kick.

Heh, it's a good sign if they do, since that means you're running really
well :)

They don't in fact, in my testing :(. But I think they can with luck.

quoted

quoted

I think block with unlocked kick can trip it too:
add, lock is dropped and then an interrupt can get.

We also don't need a kick each num - each 2^15 is enough.
Why don't we do this at start of add_buf:
if (vq->num_added >= 0x7fff)
	return -ENOSPC;

The warning was there in case a driver is never doing a kick, and
getting away with it (mostly) because the device is polling.  Let's not
penalize good drivers to catch bad ones.

How about we do this properly, like so:

Absolutely. But I think we also need to handle num_added
overflow of a 15 bit counter, no? Otherwise the
vring_need_event logic might give us false negatives ....
I'm guessing we can just assume we need a kick in that case.

You're right.  Thankyou.  My immediate reaction of "make it an unsigned
long" doesn't work.

Here's the diff to what I posted before:

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

@@ -254,9 +254,10 @@ add_head:
 	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);
+	/* This is very unlikely, but theoretically possible.  Kick
+	 * just in case. */
+	if (unlikely(vq->num_added == 65535))