netdev - [PATCH net-next v7 02/26] vsock/virtio: pack struct virtio_vsock_skb

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Message-Id: <20251021-vsock-vmtest-v7-2-0661b7b6f081@meta.com>
Date: Tue, 21 Oct 2025 16:46:45 -0700
From: Bobby Eshleman <bobbyeshleman@...il.com>
To: Stefano Garzarella <sgarzare@...hat.com>, 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>, 
 Bobby Eshleman <bobbyeshleman@...il.com>
Cc: 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: [PATCH net-next v7 02/26] vsock/virtio: pack struct
 virtio_vsock_skb_cb

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(-)

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