Re: [PATCH] VIRTIO: Update the desc 'flag' fied last in packed ring.

[Michael S. Tsirkin]

On Tue, Jun 02, 2026 at 12:31:23PM +0800, yangjiale wrote:
> When a descriptor list spans across cache lines,
> updating the flag first can lead to a scenario where the device side
> perceives the flag as valid, yet the corresponding address and length
> fields remain unupdated—resulting in invalid values.
> Therefore, the flag field must be updated last.
> 
> Signed-off-by: yangjiale <yangjiale133@xxxxxxx>
> ---
>  drivers/virtio/virtio_ring.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index fbca7ce1c6bf..036b4f90d30f 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1688,6 +1688,10 @@ static inline int virtqueue_add_packed(struct vring_virtqueue *vq,
>  					     &addr, &len, premapped, attr))
>  				goto unmap_release;
>  
> +			desc[i].addr = cpu_to_le64(addr);
> +			desc[i].len = cpu_to_le32(len);
> +			desc[i].id = cpu_to_le16(id);
> +
>  			flags = cpu_to_le16(vq->packed.avail_used_flags |
>  				    (++c == total_sg ? 0 : VRING_DESC_F_NEXT) |
>  				    (n < out_sgs ? 0 : VRING_DESC_F_WRITE));
> @@ -1696,10 +1700,6 @@ static inline int virtqueue_add_packed(struct vring_virtqueue *vq,
>  			else
>  				desc[i].flags = flags;
>  
> -			desc[i].addr = cpu_to_le64(addr);
> -			desc[i].len = cpu_to_le32(len);
> -			desc[i].id = cpu_to_le16(id);
> -
>  			if (unlikely(vq->use_map_api)) {
>  				vq->packed.desc_extra[curr].addr = premapped ?
>  					DMA_MAPPING_ERROR : addr;

Good catch!  And presumably we need a write barrier then?

> -- 
> 2.25.1