netdev - [PATCH v6 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP

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Message-Id: <20251114071345.10769-4-chia-yu.chang@nokia-bell-labs.com>
Date: Fri, 14 Nov 2025 08:13:34 +0100
From: chia-yu.chang@...ia-bell-labs.com
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Cc: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
Subject: [PATCH v6 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN

From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

No functional changes.

Co-developed-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Ilpo Järvinen <ij@...nel.org>
Signed-off-by: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>

---
v6:
- Update comments.
---
 include/linux/skbuff.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ff90281ddf90..e09455cee8e3 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -671,7 +671,13 @@ enum {
 	/* This indicates the skb is from an untrusted source. */
 	SKB_GSO_DODGY = 1 << 1,
 
-	/* This indicates the tcp segment has CWR set. */
+	/* For Tx, this indicates the first TCP segment has CWR set, and any
+	 * subsequent segment in the same skb has CWR cleared. This is not
+	 * used on Rx except for virtio_net. However, because the connection
+	 * to which the segment belongs is not tracked to use RFC3168 or
+	 * Accurate ECN, and using RFC3168 ECN offload may corrupt AccECN
+	 * signal of AccECN segments. Therefore, this cannot be used on Rx.
+	 */
 	SKB_GSO_TCP_ECN = 1 << 2,
 
 	__SKB_GSO_TCP_FIXEDID = 1 << 3,
@@ -706,6 +712,12 @@ enum {
 
 	SKB_GSO_FRAGLIST = 1 << 18,
 
+	/* For TX, this indicates the TCP segment uses the CWR flag as part of
+	 * AccECN signal, and the CWR flag is not modified in the skb. This is
+	 * not used on Rx except for virtio_net. For RX, any CWR flagged segment
+	 * must use SKB_GSO_TCP_ACCECN to ensure CWR flag is not cleared by any
+	 * RFC3168 ECN offload, and thus keeping AccECN signal of TCP segments.
+	 */
 	SKB_GSO_TCP_ACCECN = 1 << 19,
 
 	/* These indirectly map onto the same netdev feature.
-- 
2.34.1