2 files changed, 36 insertions, 0 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a5fadc40c448..68b9136847af 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -376,6 +376,33 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb);
 
+bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
+{
+        struct vring_virtqueue *vq = to_vvq(_vq);
+        u16 bufs;
+
+        START_USE(vq);
+
+        /* We optimistically turn back on interrupts, then check if there was
+         * more to do. */
+        /* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
+         * either clear the flags bit or point the event index at the next
+         * entry. Always do both to keep code simple. */
+        vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
+        /* TODO: tune this threshold */
+        bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4;
+        vring_used_event(&vq->vring) = vq->last_used_idx + bufs;
+        virtio_mb();
+        if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) {
+                END_USE(vq);
+                return false;
+        }
+
+        END_USE(vq);
+        return true;
+}
+EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
+
 void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 {
         struct vring_virtqueue *vq = to_vvq(_vq);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index aff5b4f74041..710885749605 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -51,6 +51,13 @@ struct virtqueue {
  *      This re-enables callbacks; it returns "false" if there are pending
  *      buffers in the queue, to detect a possible race between the driver
  *      checking for more work, and enabling callbacks.
+ * virtqueue_enable_cb_delayed: restart callbacks after disable_cb.
+ *      vq: the struct virtqueue we're talking about.
+ *      This re-enables callbacks but hints to the other side to delay
+ *      interrupts until most of the available buffers have been processed;
+ *      it returns "false" if there are many pending buffers in the queue,
+ *      to detect a possible race between the driver checking for more work,
+ *      and enabling callbacks.
  * virtqueue_detach_unused_buf: detach first unused buffer
  *      vq: the struct virtqueue we're talking about.
  *      Returns NULL or the "data" token handed to add_buf
@@ -86,6 +93,8 @@ void virtqueue_disable_cb(struct virtqueue *vq);
 
 bool virtqueue_enable_cb(struct virtqueue *vq);
 
+bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
+
 void *virtqueue_detach_unused_buf(struct virtqueue *vq);
 
 /**