linux-kernel - [PATCH V4 14/19] virtio_ring: determine descriptor flags at one time

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Message-ID: <20250724064017.26058-15-jasowang@redhat.com>
Date: Thu, 24 Jul 2025 14:40:12 +0800
From: Jason Wang <jasowang@...hat.com>
To: mst@...hat.com,
	jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH V4 14/19] virtio_ring: determine descriptor flags at one time

Let's determine the last descriptor by counting the number of sg. This
would be consistent with packed virtqueue implementation and ease the
future in-order implementation.

Acked-by: Eugenio Pérez <eperezma@...hat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Signed-off-by: Jason Wang <jasowang@...hat.com>
---
 drivers/virtio/virtio_ring.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 2f0b14940e05..01bbbcd65417 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -570,7 +570,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 	struct vring_desc_extra *extra;
 	struct scatterlist *sg;
 	struct vring_desc *desc;
-	unsigned int i, n, avail, descs_used, prev, err_idx;
+	unsigned int i, n, avail, descs_used, err_idx, c = 0;
 	int head;
 	bool indirect;
 
@@ -627,6 +627,7 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 	}
 
 	for (n = 0; n < out_sgs; n++) {
+		sg = sgs[n];
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
 			dma_addr_t addr;
 			u32 len;
@@ -634,12 +635,12 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 			if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
 			/* Note that we trust indirect descriptor
 			 * table since it use stream DMA mapping.
 			 */
 			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
-						     VRING_DESC_F_NEXT,
+						     ++c == total_sg ?
+						     0 : VRING_DESC_F_NEXT,
 						     premapped);
 		}
 	}
@@ -651,21 +652,15 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 			if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
 			/* Note that we trust indirect descriptor
 			 * table since it use stream DMA mapping.
 			 */
-			i = virtqueue_add_desc_split(vq, desc, extra, i, addr, len,
-						     VRING_DESC_F_NEXT |
-						     VRING_DESC_F_WRITE,
-						     premapped);
+			i = virtqueue_add_desc_split(vq, desc, extra,
+				i, addr, len,
+				(++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
+				VRING_DESC_F_WRITE, premapped);
 		}
 	}
-	/* Last one doesn't continue. */
-	desc[prev].flags &= cpu_to_virtio16(vq->vq.vdev, ~VRING_DESC_F_NEXT);
-	if (!indirect && vring_need_unmap_buffer(vq, &extra[prev]))
-		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
-			~VRING_DESC_F_NEXT;
 
 	if (indirect) {
 		/* Now that the indirect table is filled in, map it. */
-- 
2.39.5