[RFC 4/5] virtio: get desc id in order

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

If in order feature negotiated, we can skip the used ring to get
buffer's desc id sequentially.

Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
---
 drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a5ec724c0..4d57a4edc 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -144,6 +144,9 @@ struct vring_virtqueue {
 			/* DMA address and size information */
 			dma_addr_t queue_dma_addr;
 			size_t queue_size_in_bytes;
+
+			/* In order feature batch begin here */
+			u16 next_batch_desc_begin;
 		} split;
 
 		/* Available for packed ring */
@@ -700,8 +703,10 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
 	}
 
 	vring_unmap_one_split(vq, i);
-	vq->split.desc_extra[i].next = vq->free_head;
-	vq->free_head = head;
+	if (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {
+		vq->split.desc_extra[i].next = vq->free_head;
+		vq->free_head = head;
+	}
 
 	/* Plus final descriptor */
 	vq->vq.num_free++;
@@ -743,7 +748,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	void *ret;
-	unsigned int i;
+	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
+	unsigned int i, j;
 	u16 last_used;
 
 	START_USE(vq);
@@ -762,11 +768,24 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
 	/* Only get used array entries after they have been exposed by host. */
 	virtio_rmb(vq->weak_barriers);
 
-	last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
-	i = virtio32_to_cpu(_vq->vdev,
-			vq->split.vring.used->ring[last_used].id);
-	*len = virtio32_to_cpu(_vq->vdev,
-			vq->split.vring.used->ring[last_used].len);
+	if (virtio_has_feature(_vq->vdev, VIRTIO_F_IN_ORDER)) {
+		/* Skip used ring and get used desc in order*/
+		i = vq->split.next_batch_desc_begin;
+		j = i;
+		while (vq->split.vring.desc[j].flags & nextflag)
+			j = (j + 1) % vq->split.vring.num;
+		/* move to next */
+		j = (j + 1) % vq->split.vring.num;
+		vq->split.next_batch_desc_begin = j;
+
+		/* TODO: len of buffer */
+	} else {
+		last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
+		i = virtio32_to_cpu(_vq->vdev,
+				    vq->split.vring.used->ring[last_used].id);
+		*len = virtio32_to_cpu(_vq->vdev,
+				       vq->split.vring.used->ring[last_used].len);
+	}
 
 	if (unlikely(i >= vq->split.vring.num)) {
 		BAD_RING(vq, "id %u out of range\n", i);
@@ -2234,6 +2253,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
 	vq->split.avail_flags_shadow = 0;
 	vq->split.avail_idx_shadow = 0;
 
+	vq->split.next_batch_desc_begin = 0;
+
 	/* No callback?  Tell other side not to bother us. */
 	if (!callback) {
 		vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
-- 
2.17.1