netdev - [PATCH RFC bpf-next 07/20] xdp: Track if metadata is supported in xdp_frame <> xdp

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Message-Id: <20250305-afabre-traits-010-rfc2-v1-7-d0ecfb869797@cloudflare.com>
Date: Wed, 05 Mar 2025 15:32:04 +0100
From: arthur@...hurfabre.com
To: netdev@...r.kernel.org, bpf@...r.kernel.org
Cc: jakub@...udflare.com, hawk@...nel.org, yan@...udflare.com, 
 jbrandeburg@...udflare.com, thoiland@...hat.com, lbiancon@...hat.com, 
 Arthur Fabre <afabre@...udflare.com>
Subject: [PATCH RFC bpf-next 07/20] xdp: Track if metadata is supported in
 xdp_frame <> xdp_buff conversions

From: Arthur Fabre <afabre@...udflare.com>

xdp_buff stores whether metadata is supported by a NIC by setting
data_meta to be greater than data.

But xdp_frame only stores the metadata size (as metasize), so converting
between xdp_frame and xdp_buff is lossy.

Steal an unused bit in xdp_frame to track whether metadata is supported
or not.

This will lets us have "generic" functions for setting skb fields from
either xdp_frame or xdp_buff from drivers.

Signed-off-by: Arthur Fabre <afabre@...udflare.com>
---
 include/net/xdp.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp.h b/include/net/xdp.h
index 58019fa299b56dbd45c104fdfa807f73af6e4fa4..84afe07d09efdb2ab0cb78b904f02cb74f9a56b6 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -116,6 +116,9 @@ static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
 	xdp->flags |= XDP_FLAGS_FRAGS_PF_MEMALLOC;
 }
 
+static bool xdp_data_meta_unsupported(const struct xdp_buff *xdp);
+static void xdp_set_data_meta_invalid(struct xdp_buff *xdp);
+
 static __always_inline void *xdp_buff_traits(const struct xdp_buff *xdp)
 {
 	return xdp->data_hard_start + _XDP_FRAME_SIZE;
@@ -270,7 +273,9 @@ struct xdp_frame {
 	void *data;
 	u32 len;
 	u32 headroom;
-	u32 metasize; /* uses lower 8-bits */
+	u32	:23, /* unused */
+		meta_unsupported:1,
+		metasize:8;
 	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
 	 * while mem_type is valid on remote CPU.
 	 */
@@ -369,6 +374,8 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
 	xdp->data = frame->data;
 	xdp->data_end = frame->data + frame->len;
 	xdp->data_meta = frame->data - frame->metasize;
+	if (frame->meta_unsupported)
+		xdp_set_data_meta_invalid(xdp);
 	xdp->frame_sz = frame->frame_sz;
 	xdp->flags = frame->flags;
 }
@@ -396,6 +403,7 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
 	xdp_frame->len  = xdp->data_end - xdp->data;
 	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
 	xdp_frame->metasize = metasize;
+	xdp_frame->meta_unsupported = xdp_data_meta_unsupported(xdp);
 	xdp_frame->frame_sz = xdp->frame_sz;
 	xdp_frame->flags = xdp->flags;
 

-- 
2.43.0