Re: [PATCH] virtio_rpmsg: fix config space alignment

[Mathieu Poirier <mathieu.poirier@linaro.org>]

On Sun, Nov 01, 2026 at 02:54:03PM +0100, Francesco Valla wrote:
> The comment for virtio_rpmsg_config says that this structure is
> naturally aligned to 32 bits, and this is desirable by some
> architectures that don't get along with unaligned accesses.
> The content of the structure itself however isn't aligned, and the
> __packed attribute prevent the compiler to add this alignment.
> 
> Fix this by adding a padding byte.
> 
> Fixes: ca96ea25e51a ("rpmsg: virtio_rpmsg_bus: Get buffer size from config space")
> Signed-off-by: Francesco Valla <francesco@valla.it>
> ---
> Hello,
> 
> unfortunately I missed this when the original patch set was sent, but I
> think it's worth fixing (or at least discussing) this now before it
> enters a stable kernel.
> 
> The same support in open-amp has already been merged [1], but maybe it
> can be fixed before the next release there too?
> 
> Another, lower-impact solution would be to fix the comment, but that can
> then led to issues on architectures that does not support unaligned
> accesses (e.g.: AMR Cortex-M0).
>

Thanks for bringing this to my attention.  I agree with this patch.  I will give
Arnaud a couple of weeks to reply to this email though.
 
> Thank you
> 
> Francesco
> 
> [1] https://github.com/OpenAMP/open-amp/commit/744376f566c3ec7e06a7d093d785295b78aa486f
> ---
>  include/linux/rpmsg/virtio_rpmsg.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/rpmsg/virtio_rpmsg.h b/include/linux/rpmsg/virtio_rpmsg.h
> index 0872cbca4593..d24fedb63023 100644
> --- a/include/linux/rpmsg/virtio_rpmsg.h
> +++ b/include/linux/rpmsg/virtio_rpmsg.h
> @@ -23,6 +23,7 @@
>   *
>   * @version:	version of this structure, currently %RPMSG_VDEV_CONFIG_V1.
>   * @size:	size of this structure in bytes.
> + * @reserved:	padding, reserved for future usage.
>   * @txbuf_size:	Tx buf size from remote's view. For Linux this is rx buf size.
>   * @rxbuf_size:	Rx buf size from remote's view. For Linux this is tx buf size.
>   *
> @@ -33,6 +34,7 @@
>  struct virtio_rpmsg_config {
>  	u8 version;
>  	__virtio16 size;
> +	u8 reserved;
>  	/* The tx/rx individual buffer size (if VIRTIO_RPMSG_F_BUFSZ) */
>  	__virtio32 txbuf_size;
>  	__virtio32 rxbuf_size;
> 
> ---
> base-commit: cbbddb6b08cc9900a87cccaa9b22bbe06aa4f50a
> change-id: 20261101-rpmsg-config-size-0dbe9ea63c6d
> 
> Best regards,
> --  
> Francesco Valla <francesco@valla.it>
>