Re: [RFC v2 6/7] virtio: in order support for virtio_ring

----- Original Message -----

From: "Xuan Zhuo" <xuanzhuo@linux.alibaba.com>
To: "Guo Zhi" <redacted>
Cc: "netdev" <redacted>, "linux-kernel" <redacted>, "kvm list" <redacted>,
"virtualization" [off-list ref], "Guo Zhi" [off-list ref], "eperezma"
[off-list ref], "jasowang" [off-list ref], "sgarzare" [off-list ref], "Michael Tsirkin"
[off-list ref]
Sent: Thursday, August 18, 2022 11:11:58 AM
Subject: Re: [RFC v2 6/7] virtio: in order support for virtio_ring

On Wed, 17 Aug 2022 21:57:17 +0800, Guo Zhi [off-list ref] wrote:

quoted

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

Signed-off-by: Guo Zhi <redacted>
---
 drivers/virtio/virtio_ring.c | 53 ++++++++++++++++++++++++++++++------
 1 file changed, 45 insertions(+), 8 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 1c1b3fa376a2..143184ebb5a1 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_desc_begin;
 		} split;

 		/* Available for packed ring */

@@ -702,8 +705,13 @@ 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;
+	/* In order feature use desc in order,
+	 * that means, the next desc will always be free
+	 */
+	if (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {

Call virtio_has_feature() here is not good.

Thanks.

Maybe I can use a variable like vq->indiret?
Thanks.

quoted

+		vq->split.desc_extra[i].next = vq->free_head;
+		vq->free_head = head;
+	}

 	/* Plus final descriptor */
 	vq->vq.num_free++;

@@ -745,7 +753,7 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue

*_vq,
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 	void *ret;
-	unsigned int i;
+	unsigned int i, j;
 	u16 last_used;

 	START_USE(vq);

@@ -764,11 +772,38 @@ 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_desc_begin;
+		j = i;
+		/* Indirect only takes one descriptor in descriptor table */
+		while (!vq->indirect && (vq->split.desc_extra[j].flags & VRING_DESC_F_NEXT))
+			j = (j + 1) % vq->split.vring.num;
+		/* move to next */
+		j = (j + 1) % vq->split.vring.num;
+		/* Next buffer will use this descriptor in order */
+		vq->split.next_desc_begin = j;
+		if (!vq->indirect) {
+			*len = vq->split.desc_extra[i].len;
+		} else {
+			struct vring_desc *indir_desc =
+				vq->split.desc_state[i].indir_desc;
+			u32 indir_num = vq->split.desc_extra[i].len, buffer_len = 0;
+
+			if (indir_desc) {
+				for (j = 0; j < indir_num / sizeof(struct vring_desc); j++)
+					buffer_len += indir_desc[j].len;
+			}
+
+			*len = buffer_len;
+		}
+	} 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);

@@ -2236,6 +2271,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int

index,
 	vq->split.avail_flags_shadow = 0;
 	vq->split.avail_idx_shadow = 0;

+	vq->split.next_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