Am Donnerstag, 19. Juni 2008 schrieb Rusty Russell:

We force notification when the ring is full, even if the host has
indicated it doesn't want to know.  This seemed like a good idea at
the time: if we fill the transmit ring, we should tell the host
immediately.

Unfortunately this logic also applies to the receiving ring, which is
refilled constantly.  We should introduce real notification thesholds
to replace this logic.  Meanwhile, removing the logic altogether breaks
the heuristics which KVM uses, so we use a hack: only notify if there are
outgoing parts of the new buffer.

I did not yet finish the VIRTIO_F_NOTIFY_ON_EMPTY implementation for s390, so 
I cant tell for sure if it makes a difference.

Nevertheless, the patch looks good to me.