[PATCH 1/3] virtio: VIRTIO_F_NOTIFY_ON_EMPTY to force callback on empty

[Rusty Russell <rusty@rustcorp.com.au>]

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>
---
 drivers/virtio/virtio_ring.c  |    7 -------
 include/linux/virtio_config.h |    4 ++++
 2 files changed, 4 insertions(+), 7 deletions(-)

diff -r 95a02f0e0e21 drivers/virtio/virtio_ring.c
--- a/drivers/virtio/virtio_ring.c	Tue May 27 12:45:17 2008 +1000
+++ b/drivers/virtio/virtio_ring.c	Tue May 27 15:53:41 2008 +1000
@@ -253,13 +253,6 @@ irqreturn_t vring_interrupt(int irq, voi
 	if (unlikely(vq->broken))
 		return IRQ_HANDLED;
 
-	/* Other side may have missed us turning off the interrupt,
-	 * but we should preserve disable semantic for virtio users. */
-	if (unlikely(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) {
-		pr_debug("virtqueue interrupt after disable for %p\n", vq);
-		return IRQ_HANDLED;
-	}
-
 	pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
 	if (vq->vq.callback)
 		vq->vq.callback(&vq->vq);
diff -r 95a02f0e0e21 include/linux/virtio_config.h
--- a/include/linux/virtio_config.h	Tue May 27 12:45:17 2008 +1000
+++ b/include/linux/virtio_config.h	Tue May 27 15:53:41 2008 +1000
@@ -14,6 +14,10 @@
 #define VIRTIO_CONFIG_S_DRIVER_OK	4
 /* 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>