linux-kernel - Re: [PATCH net-next v8 02/14] vsock/virtio: pack struct virtio_vsock_skb

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Message-ID: <ruxocfuprofj3mktmjulqy5dhnzkbad3fetqrg2f6kw4gh4wwj@x2mb2dw7pjk5>
Date: Thu, 6 Nov 2025 17:16:46 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Bobby Eshleman <bobbyeshleman@...il.com>
Cc: Shuah Khan <shuah@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Stefan Hajnoczi <stefanha@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Jason Wang <jasowang@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Eugenio Pérez <eperezma@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com>, 
	Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Bryan Tan <bryan-bt.tan@...adcom.com>, Vishnu Dasa <vishnu.dasa@...adcom.com>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-hyperv@...r.kernel.org, berrange@...hat.com, Bobby Eshleman <bobbyeshleman@...a.com>
Subject: Re: [PATCH net-next v8 02/14] vsock/virtio: pack struct
 virtio_vsock_skb_cb

On Thu, Oct 23, 2025 at 11:27:41AM -0700, Bobby Eshleman wrote:
>From: Bobby Eshleman <bobbyeshleman@...a.com>
>
>Reduce holes in struct virtio_vsock_skb_cb. As this struct continues to
>grow, we want to keep it trimmed down so it doesn't exceed the size of
>skb->cb (currently 48 bytes). Eliminating the 2 byte hole provides an
>additional two bytes for new fields at the end of the structure. It does
>not shrink the total size, however.
>
>Future work could include combining fields like reply and tap_delivered
>into a single bitfield, but currently doing so will not make the total
>struct size smaller (although, would extend the tail-end padding area by
>one byte).
>
>Before this patch:
>
>struct virtio_vsock_skb_cb {
>	bool                       reply;                /*     0     1 */
>	bool                       tap_delivered;        /*     1     1 */
>
>	/* XXX 2 bytes hole, try to pack */
>
>	u32                        offset;               /*     4     4 */
>
>	/* size: 8, cachelines: 1, members: 3 */
>	/* sum members: 6, holes: 1, sum holes: 2 */
>	/* last cacheline: 8 bytes */
>};
>;
>
>After this patch:
>
>struct virtio_vsock_skb_cb {
>	u32                        offset;               /*     0     4 */
>	bool                       reply;                /*     4     1 */
>	bool                       tap_delivered;        /*     5     1 */
>
>	/* size: 8, cachelines: 1, members: 3 */
>	/* padding: 2 */
>	/* last cacheline: 8 bytes */
>};
>
>Signed-off-by: Bobby Eshleman <bobbyeshleman@...a.com>
>---
> include/linux/virtio_vsock.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)

Yeah, thanks for that!

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

>
>diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
>index 0c67543a45c8..87cf4dcac78a 100644
>--- a/include/linux/virtio_vsock.h
>+++ b/include/linux/virtio_vsock.h
>@@ -10,9 +10,9 @@
> #define VIRTIO_VSOCK_SKB_HEADROOM (sizeof(struct virtio_vsock_hdr))
>
> struct virtio_vsock_skb_cb {
>+	u32 offset;
> 	bool reply;
> 	bool tap_delivered;
>-	u32 offset;
> };
>
> #define VIRTIO_VSOCK_SKB_CB(skb) ((struct virtio_vsock_skb_cb *)((skb)->cb))
>
>-- 
>2.47.3
>