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

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Message-ID: <20250324060127.2358-2-jasowang@redhat.com>
Date: Mon, 24 Mar 2025 14:01:22 +0800
From: Jason Wang <jasowang@...hat.com>
To: mst@...hat.com,
	jasowang@...hat.com
Cc: xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH 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.

Signed-off-by: Jason Wang <jasowang@...hat.com>
---
 drivers/virtio/virtio_ring.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index ce1dc90ee89d..31aa4a935c27 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -561,7 +561,7 @@ 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, c, avail, descs_used, err_idx;
 	int head;
 	bool indirect;
 
@@ -617,46 +617,47 @@ static inline int virtqueue_add_split(struct vring_virtqueue *vq,
 		return -ENOSPC;
 	}
 
+	c = 0;
 	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;
+			u16 flags = 0;
 
 			if (vring_map_one_sg(vq, sg, DMA_TO_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
+			if (++c != total_sg)
+				flags = VRING_DESC_F_NEXT;
+
 			/* 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,
+						     flags,
 						     premapped);
 		}
 	}
 	for (; n < (out_sgs + in_sgs); n++) {
 		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
+			u16 flags = VRING_DESC_F_WRITE;
 			dma_addr_t addr;
 			u32 len;
 
 			if (vring_map_one_sg(vq, sg, DMA_FROM_DEVICE, &addr, &len, premapped))
 				goto unmap_release;
 
-			prev = i;
+			if (++c != total_sg)
+				flags |= VRING_DESC_F_NEXT;
+
 			/* 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);
+						     flags, 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. */
-- 
2.42.0