virtio: force callback on empty.

virtio allows drivers to suppress callbacks (ie. interrupts) for
efficiency (no locking, it's just an optimization).

There's a similar mechanism for the host to suppress notifications
coming from the guest: in that case, we ignore the suppression if the
ring is completely full.

It turns out that life is simpler if the host similarly ignores
callback suppression when the ring is completely empty: the network
driver wants to free up old packets in a timely manner, and otherwise
has to use a timer to poll.

We have to remove the code which ignores interrupts when the driver
has disabled them (again, it had no locking and hence was unreliable
anyway).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_config.h

@@ -15,6 +15,10 @@
 /* We've given up on this device. */
 #define VIRTIO_CONFIG_S_FAILED		0x80
 
+/* Do we get callbacks when the ring is completely used, even if we've
+ * suppressed them? */
+#define VIRTIO_F_NOTIFY_ON_EMPTY	24
+
 #ifdef __KERNEL__
 #include <linux/virtio.h>