quoted

drivers decide how they use it. Drivers can do BUILD_BUG_ON() for their
expected size and cast that to whatever struct they want. This is how
various offloads work, the variable size tailroom would be an over
design IMO.

quoted

And this way non XSK paths can keep its normal typing.

Good idea, prototyped below, lmk if it that's not what you had in mind.

struct xdp_buff_xsk {
	struct xdp_buff            xdp;                  /*     0    56 */
	u8                         cb[16];               /*    56    16 */
	/* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
	dma_addr_t                 dma;                  /*    72     8 */
	dma_addr_t                 frame_dma;            /*    80     8 */
	struct xsk_buff_pool *     pool;                 /*    88     8 */
	u64                        orig_addr;            /*    96     8 */
	struct list_head           free_list_node;       /*   104    16 */

	/* size: 120, cachelines: 2, members: 7 */
	/* last cacheline: 56 bytes */
};

Toke, I can try to merge this into your patch + keep your SoB (or feel free
to try this and retest yourself, whatever works).

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h

b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h
index bc2d9034af5b..837bf103b871 100644

--- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.h

@@ -44,6 +44,11 @@
 	(MLX5E_XDP_INLINE_WQE_MAX_DS_CNT * MLX5_SEND_WQE_DS - \
 	 sizeof(struct mlx5_wqe_inline_seg))

+struct mlx5_xdp_cb {
+	struct mlx5_cqe64 *cqe;
+	struct mlx5e_rq *rq;
+};
+
 struct mlx5e_xsk_param;
 int mlx5e_xdp_max_mtu(struct mlx5e_params *params, struct mlx5e_xsk_param

*xsk);
 bool mlx5e_xdp_handle(struct mlx5e_rq *rq, struct page *page,

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c

b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
index c91b54d9ff27..84d23b2da7ce 100644

--- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/rx.c

@@ -5,6 +5,7 @@
 #include "en/xdp.h"
 #include <net/xdp_sock_drv.h>
 #include <linux/filter.h>
+#include <linux/build_bug.h>

 /* RX data path */

@@ -286,8 +287,14 @@ struct sk_buff *mlx5e_xsk_skb_from_cqe_linear(struct

mlx5e_rq *rq,
 					      u32 cqe_bcnt)
 {
 	struct xdp_buff *xdp = wi->au->xsk;
+	struct mlx5_xdp_cb *cb;
 	struct bpf_prog *prog;

+	BUILD_BUG_ON(sizeof(struct mlx5_xdp_cb) > XSKB_CB_SIZE);
+	cb = xp_get_cb(xdp);
+	cb->cqe = NULL /*cqe*/;
+	cb->rq = rq;

+
 	/* wi->offset is not used in this function, because xdp->data and the
 	 * DMA address point directly to the necessary place. Furthermore, the
 	 * XSK allocator allocates frames per packet, instead of pages, so

diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
index f787c3f524b0..b298590429e7 100644
--- a/include/net/xsk_buff_pool.h
+++ b/include/net/xsk_buff_pool.h

@@ -19,8 +19,11 @@ struct xdp_sock;
 struct device;
 struct page;

+#define XSKB_CB_SIZE 16
+
 struct xdp_buff_xsk {
 	struct xdp_buff xdp;
+	u8 cb[XSKB_CB_SIZE]; /* Private area for the drivers to use. */
 	dma_addr_t dma;
 	dma_addr_t frame_dma;
 	struct xsk_buff_pool *pool;

@@ -143,6 +146,11 @@ static inline dma_addr_t xp_get_frame_dma(struct

xdp_buff_xsk *xskb)
 	return xskb->frame_dma;
 }

+static inline void *xp_get_cb(struct xdp_buff *xdp)
+{
+	return (void *)xdp + offsetof(struct xdp_buff_xsk, cb);
+}