[RFC RESEND 1/3] virtio: add virtqueue_get_last_used_idx() helper
From: Longjun Tang <hidden>
Date: 2026-09-08 09:27:19
Also in:
netdev
Subsystem:
the rest, virtio core · Maintainers:
Linus Torvalds, "Michael S. Tsirkin", Jason Wang, Eugenio Pérez
From: Longjun Tang <redacted> Export a read-only accessor for the last consumed used ring index. It's useful for drivers that need to detect whether the device has produced buffers that have not been consumed yet, by passing the returned value to virtqueue_poll(). The split ring now writes the field with WRITE_ONCE() (the packed ring already did) to document that the new reader can run concurrently with the poll path. Assisted-by: Kilo:deepseek-v4-pro Signed-off-by: Longjun Tang <redacted> --- drivers/virtio/virtio_ring.c | 18 ++++++++++++++++-- include/linux/virtio.h | 2 ++ 2 files changed, 18 insertions(+), 2 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 5c169fbb418a..f29ef246cff8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c@@ -1001,7 +1001,7 @@ static void *virtqueue_get_buf_ctx_split(struct vring_virtqueue *vq, /* detach_buf_split clears data, so grab it now. */ ret = vq->split.desc_state[i].data; detach_buf_split(vq, i, ctx); - vq->last_used_idx++; + WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1); /* If we expect an interrupt for the next entry, tell host * by writing event index and flush out the write before * the read in the next get_buf call. */
@@ -1068,7 +1068,7 @@ static void *virtqueue_get_buf_ctx_split_in_order(struct vring_virtqueue *vq, ret = vq->split.desc_state[last_used].data; detach_buf_split_in_order(vq, last_used, ctx); - vq->last_used_idx++; + WRITE_ONCE(vq->last_used_idx, vq->last_used_idx + 1); vq->last_used += (vq->vq.num_free - num_free); /* If we expect an interrupt for the next entry, tell host * by writing event index and flush out the write before
@@ -3197,6 +3197,20 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx) } EXPORT_SYMBOL_GPL(virtqueue_poll); +/** + * virtqueue_get_last_used_idx - get the last consumed used ring index + * @_vq: the struct virtqueue we're talking about. + * + * Returns the index of the most recently consumed used buffer. + */ +unsigned int virtqueue_get_last_used_idx(const struct virtqueue *_vq) +{ + const struct vring_virtqueue *vq = to_vvq(_vq); + + return READ_ONCE(vq->last_used_idx); +} +EXPORT_SYMBOL_GPL(virtqueue_get_last_used_idx); + /** * virtqueue_enable_cb - restart callbacks after disable_cb. * @_vq: the struct virtqueue we're talking about.
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index f923e42cfd01..1aed4d9235c8 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h@@ -112,6 +112,8 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq); bool virtqueue_poll(struct virtqueue *vq, unsigned); +unsigned int virtqueue_get_last_used_idx(const struct virtqueue *vq); + bool virtqueue_enable_cb_delayed(struct virtqueue *vq); void *virtqueue_detach_unused_buf(struct virtqueue *vq);
--
2.25.1