[PATCH v3] virtio_ring: use shadow flags when detaching split descriptors

[sung byeongchan <tjdqudcks0424@naver.com>]

From: sungbyeongchan <tjdqudcks0424@naver.com>

Split virtqueues save descriptor flags and next indexes in desc_extra
before publishing descriptors to the device. The detach path uses the
shadow next index, but decides whether to continue by rereading the NEXT
flag from the shared descriptor.

Use the shadow flags for the continuation decision as well. This keeps
all detach metadata in the same driver-owned snapshot and avoids an
unnecessary shared-ring read.

Fixes: 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra for split virtqueue")
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: LLM
Signed-off-by: sungbyeongchan <tjdqudcks0424@naver.com>
---
Changes in v3:
- Restore a concise commit message explaining the mixed shared/shadow
  metadata problem. The v2 response had removed the whole description.
- Keep Michael's Acked-by.
- No code changes.

Changes in v2:
- Drop the two paragraphs Michael identified as unnecessary.
- Add Michael's Acked-by.

 drivers/virtio/virtio_ring.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index db678f5a80e03..b4cb433df1dec 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -905,7 +905,6 @@ static unsigned detach_buf_split_in_order(struct vring_virtqueue *vq,
 {
 	struct vring_desc_extra *extra;
 	unsigned int i;
-	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
 
 	/* Clear data ptr. */
 	vq->split.desc_state[head].data = NULL;
@@ -915,7 +914,7 @@ static unsigned detach_buf_split_in_order(struct vring_virtqueue *vq,
 	/* Put back on free list: unmap first-level descriptors and find end */
 	i = head;
 
-	while (vq->split.vring.desc[i].flags & nextflag) {
+	while (extra[i].flags & VRING_DESC_F_NEXT) {
 		i = vring_unmap_one_split(vq, &extra[i]);
 		vq->vq.num_free++;
 	}
base-commit: 0c2669a9f4a1d607e7591ae50ccf3c432a0aff08
-- 
2.43.0