Re: [PATCH] virtio: Reorder fields in 'struct virtqueue'

[Stefano Garzarella]

On Sat, Feb 18, 2023 at 09:10:31AM +0100, Christophe JAILLET wrote:
> Group some variables based on their sizes to reduce hole and avoid padding.
> On x86_64, this shrinks the size of 'struct virtqueue'
> from 72 to 68 bytes.
>
> It saves a few bytes of memory.
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> ---

LGTM!

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>

> Using pahole
>
> Before:
> ======
> struct virtqueue {
> 	struct list_head           list;                 /*     0    16 */
> 	void                       (*callback)(struct virtqueue *); /*    16     8 */
> 	const char  *              name;                 /*    24     8 */
> 	struct virtio_device *     vdev;                 /*    32     8 */
> 	unsigned int               index;                /*    40     4 */
> 	unsigned int               num_free;             /*    44     4 */
> 	unsigned int               num_max;              /*    48     4 */
>
> 	/* XXX 4 bytes hole, try to pack */
>
> 	void *                     priv;                 /*    56     8 */
> 	/* --- cacheline 1 boundary (64 bytes) --- */
> 	bool                       reset;                /*    64     1 */
>
> 	/* size: 72, cachelines: 2, members: 9 */
> 	/* sum members: 61, holes: 1, sum holes: 4 */
> 	/* padding: 7 */
> 	/* last cacheline: 8 bytes */
> };
>
> After:
> =====
> struct virtqueue {
> 	struct list_head           list;                 /*     0    16 */
> 	void                       (*callback)(struct virtqueue *); /*    16     8 */
> 	const char  *              name;                 /*    24     8 */
> 	struct virtio_device *     vdev;                 /*    32     8 */
> 	unsigned int               index;                /*    40     4 */
> 	unsigned int               num_free;             /*    44     4 */
> 	unsigned int               num_max;              /*    48     4 */
> 	bool                       reset;                /*    52     1 */
>
> 	/* XXX 3 bytes hole, try to pack */
>
> 	void *                     priv;                 /*    56     8 */
>
> 	/* size: 64, cachelines: 1, members: 9 */
> 	/* sum members: 61, holes: 1, sum holes: 3 */
> };
> ---
> include/linux/virtio.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 6ac2655500dc..9439ae898310 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -35,8 +35,8 @@ struct virtqueue {
> 	unsigned int index;
> 	unsigned int num_free;
> 	unsigned int num_max;
> -	void *priv;
> 	bool reset;
> +	void *priv;
> };
>
> int virtqueue_add_outbuf(struct virtqueue *vq,
> --
> 2.34.1