[PATCH 4/9] kvm: qemu: Add VIRTIO_F_NOTIFY_ON_EMPTY

[Mark McLoughlin <markmc@redhat.com>]

Set the VIRTIO_F_NOTIFY_ON_EMPTY feature bit so the
guest can rely on us notifying them when the queue
is empty.

Also, only notify when the available queue is empty
*and* when we've finished with all the buffers we
had detached. Right now, when the queue is empty,
we notify the guest for every used buffer.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
---
 qemu/hw/virtio.c |    6 +++++-
 qemu/hw/virtio.h |    5 +++++
 2 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/qemu/hw/virtio.c b/qemu/hw/virtio.c
index 3429ac8..e035e4e 100644
--- a/qemu/hw/virtio.c
+++ b/qemu/hw/virtio.c
@@ -138,6 +138,7 @@ void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
     /* Make sure buffer is written before we update index. */
     wmb();
     vq->vring.used->idx++;
+    vq->inuse--;
 }
 
 int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
@@ -187,6 +188,8 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
 
     elem->index = head;
 
+    vq->inuse++;
+
     return elem->in_num + elem->out_num;
 }
 
@@ -275,6 +278,7 @@ static uint32_t virtio_ioport_read(void *opaque, uint32_t addr)
     switch (addr) {
     case VIRTIO_PCI_HOST_FEATURES:
 	ret = vdev->get_features(vdev);
+	ret |= (1 << VIRTIO_F_NOTIFY_ON_EMPTY);
 	break;
     case VIRTIO_PCI_GUEST_FEATURES:
 	ret = vdev->features;
@@ -431,7 +435,7 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
     /* Always notify when queue is empty */
-    if (vq->vring.avail->idx != vq->last_avail_idx &&
+    if ((vq->inuse || vq->vring.avail->idx != vq->last_avail_idx) &&
 	(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
 	return;
 
diff --git a/qemu/hw/virtio.h b/qemu/hw/virtio.h
index 61f5038..1adaed3 100644
--- a/qemu/hw/virtio.h
+++ b/qemu/hw/virtio.h
@@ -30,6 +30,10 @@
 /* We've given up on this device. */
 #define VIRTIO_CONFIG_S_FAILED		0x80
 
+/* We notify when the ring is completely used, even if the guest is supressing
+ * callbacks */
+#define VIRTIO_F_NOTIFY_ON_EMPTY        24
+
 /* from Linux's linux/virtio_ring.h */
 
 /* This marks a buffer as continuing via the next field. */
@@ -86,6 +90,7 @@ struct VirtQueue
     VRing vring;
     uint32_t pfn;
     uint16_t last_avail_idx;
+    int inuse;
     void (*handle_output)(VirtIODevice *vdev, VirtQueue *vq);
 };
 
-- 
1.5.4.1