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>

2 files changed, 4 insertions, 7 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 96d2567a7df8..72bf8bc09014 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -253,13 +253,6 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
         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 --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index 71d6c102497e..f364bbf63c34 100644
--- a/include/linux/virtio_config.h
+++ b/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>