[PATCH] kvm tools: Use vring_need_event() to determine if interrupt is needed

[Asias He <asias.hejun@gmail.com>]

This patch also fixes fio seq-read hang problem.

   root@guest-kvm:~# cat seq-read.fio
   [seq-read]
   rw=read
   bs=4096
   size=512m
   direct=1
   filename=/dev/vdb

   root@guest-kvm:~# fio seq-read.fio
   random-read: (g=0): rw=read, bs=4K-4K/4K-4K, ioengine=sync, iodepth=1
   Starting 1 process
   Jobs: 1 (f=1): [R] [50.0% done] [0K/0K /s] [0/0 iops] [eta 00m:27s]

Signed-off-by: Asias He <asias.hejun@gmail.com>
---
 tools/kvm/include/kvm/virtio.h |    6 ++----
 tools/kvm/virtio/core.c        |   16 ++++++++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/tools/kvm/include/kvm/virtio.h b/tools/kvm/include/kvm/virtio.h
index cd24285..d117bfc 100644
--- a/tools/kvm/include/kvm/virtio.h
+++ b/tools/kvm/include/kvm/virtio.h
@@ -22,6 +22,7 @@ struct virt_queue {
 	/* The last_avail_idx field is an index to ->ring of struct vring_avail.
 	   It's where we assume the next request index is at.  */
 	u16		last_avail_idx;
+	u16		last_used_signalled;
 };
 
 static inline u16 virt_queue__pop(struct virt_queue *queue)
@@ -53,13 +54,10 @@ static inline void *guest_pfn_to_host(struct kvm *kvm, u32 pfn)
 	return guest_flat_to_host(kvm, (unsigned long)pfn << VIRTIO_PCI_QUEUE_ADDR_SHIFT);
 }
 
-static inline int virtio_queue__should_signal(struct virt_queue *vq)
-{
-	return vring_used_event(&vq->vring) <= vq->vring.used->idx;
-}
 
 struct vring_used_elem *virt_queue__set_used_elem(struct virt_queue *queue, u32 head, u32 len);
 
+bool virtio_queue__should_signal(struct virt_queue *vq);
 u16 virt_queue__get_iov(struct virt_queue *queue, struct iovec iov[], u16 *out, u16 *in, struct kvm *kvm);
 u16 virt_queue__get_inout_iov(struct kvm *kvm, struct virt_queue *queue,
 			      struct iovec in_iov[], struct iovec out_iov[],
diff --git a/tools/kvm/virtio/core.c b/tools/kvm/virtio/core.c
index 0466e07..8032d7a 100644
--- a/tools/kvm/virtio/core.c
+++ b/tools/kvm/virtio/core.c
@@ -109,3 +109,19 @@ int virtio__get_dev_specific_field(int offset, bool msix, bool features_hi, u32
 
 	return VIRTIO_PCI_O_CONFIG;
 }
+
+bool virtio_queue__should_signal(struct virt_queue *vq)
+{
+	u16 old_idx, new_idx, event_idx;
+
+	old_idx		= vq->last_used_signalled;
+	new_idx		= vq->vring.used->idx;
+	event_idx	= vring_used_event(&vq->vring);
+
+	if (vring_need_event(event_idx, new_idx, old_idx)) {
+		vq->last_used_signalled = new_idx;
+		return true;
+	}
+
+	return false;
+}
-- 
1.7.7.3