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SubjectRe: [PATCH] virtio: Reorder fields in 'struct virtqueue'
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@wanadoo.fr>
>---

LGTM!

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>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
>

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