[RFC v3 1/7] vhost: expose used buffers

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
of descriptors.

Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
---
 drivers/vhost/vhost.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 40097826cff0..26862c8bf751 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
 	vring_used_elem_t __user *used;
 	u16 old, new;
 	int start;
+	int copy_n = count;
 
+	/**
+	 * If in order feature negotiated, devices can notify the use of a batch of buffers to
+	 * the driver by only writing out a single used ring entry with the id corresponding
+	 * to the head entry of the descriptor chain describing the last buffer in the batch.
+	 */
+	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
+		copy_n = 1;
+		heads = &heads[count - 1];
+	}
 	start = vq->last_used_idx & (vq->num - 1);
 	used = vq->used->ring + start;
-	if (vhost_put_used(vq, heads, start, count)) {
+	if (vhost_put_used(vq, heads, start, copy_n)) {
 		vq_err(vq, "Failed to write used");
 		return -EFAULT;
 	}
@@ -2388,7 +2398,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
 		smp_wmb();
 		/* Log used ring entry write. */
 		log_used(vq, ((void __user *)used - (void __user *)vq->used),
-			 count * sizeof *used);
+			 copy_n * sizeof(*used));
 	}
 	old = vq->last_used_idx;
 	new = (vq->last_used_idx += count);
@@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
 
 	start = vq->last_used_idx & (vq->num - 1);
 	n = vq->num - start;
-	if (n < count) {
+	if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
 		r = __vhost_add_used_n(vq, heads, n);
 		if (r < 0)
 			return r;
-- 
2.17.1