[RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

[Jason Wang]

This patch introduces virtio_enable_cb_avail() to publish avail idx
and used event. This could be used by batched buffer submitting to
reduce the number of tx interrupts.

Cc: Rusty Russell <ru...@rustcorp.com.au>
Cc: Michael S. Tsirkin <m...@redhat.com>
Signed-off-by: Jason Wang <jasow...@redhat.com>
---
 drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
 include/linux/virtio.h       |    2 ++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1b3929f..d67fbf8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue 
*_vq)
         * entry. Always do both to keep code simple. */
        vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
        /* Make sure used event never go backwards */
-       if (!vring_need_event(vring_used_event(&vq->vring),
-                             vq->vring.avail->idx, last_used_idx))
+       if (vq->vring.avail->idx != vring_used_event(&vq->vring) &&
+           !vring_need_event(vring_used_event(&vq->vring),
+                             vq->vring.avail->idx, last_used_idx)) {
                vring_used_event(&vq->vring) = last_used_idx;
+       }
        END_USE(vq);
        return last_used_idx;
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
 
+bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
+{
+       struct vring_virtqueue *vq = to_vvq(_vq);
+       bool ret;
+
+       START_USE(vq);
+       vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+       vring_used_event(&vq->vring) = vq->vring.avail->idx;
+       ret = vring_need_event(vq->vring.avail->idx,
+                              vq->last_used_idx, vq->vring.used->idx);
+       END_USE(vq);
+
+       return ret;
+}
+EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
+
 /**
  * virtqueue_poll - query pending used buffers
  * @vq: the struct virtqueue we're talking about.
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index b46671e..bfaf058 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
 
 unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
 
+bool virtqueue_enable_cb_avail(struct virtqueue *vq);
+
 bool virtqueue_poll(struct virtqueue *vq, unsigned);
 
 bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
-- 
1.7.1

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