-----Original Message----- From: Paolo Abeni pabeni@redhat.com Sent: Thursday, November 6, 2025 12:26 PM To: Chia-Yu Chang (Nokia) chia-yu.chang@nokia-bell-labs.com; edumazet@google.com; parav@nvidia.com; linux-doc@vger.kernel.org; corbet@lwn.net; horms@kernel.org; dsahern@kernel.org; kuniyu@google.com; bpf@vger.kernel.org; netdev@vger.kernel.org; dave.taht@gmail.com; jhs@mojatatu.com; kuba@kernel.org; stephen@networkplumber.org; xiyou.wangcong@gmail.com; jiri@resnulli.us; davem@davemloft.net; andrew+netdev@lunn.ch; donald.hunter@gmail.com; ast@fiberby.net; liuhangbin@gmail.com; shuah@kernel.org; linux-kselftest@vger.kernel.org; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) koen.de_schepper@nokia-bell-labs.com; g.white@cablelabs.com; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com; cheshire cheshire@apple.com; rs.ietf@gmx.at; Jason_Livingood@comcast.com; Vidhi Goel vidhi_goel@apple.com Subject: Re: [PATCH v5 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN
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On 11/6/25 12:06 PM, Paolo Abeni wrote:
On 10/30/25 3:34 PM, chia-yu.chang@nokia-bell-labs.com wrote:
From: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com
No functional changes.
Co-developed-by: Ilpo Järvinen ij@kernel.org Signed-off-by: Ilpo Järvinen ij@kernel.org Signed-off-by: Chia-Yu Chang chia-yu.chang@nokia-bell-labs.com
include/linux/skbuff.h | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index a7cc3d1f4fd1..74d6a209e203 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -671,7 +671,12 @@ 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 cannot be* used on Rx, 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.*/The intended difference between RX and TX sounds bad to me; I think it conflicts with the basic GRO design goal of making aggregated and re-segmented traffic indistinguishable from the original stream. Also what about forwarded packet?
Uhm... I missed completely the point that SKB_GSO_TCP_ECN is TX path only, i.e. GRO never produces aggregated SKB_GSO_TCP_ECN packets. Except virtio_net uses it in the RX path ( virtio_net_hdr_to_skb ). Please clarify the statement accordingly.
/P
Hi Paolo,
Yes, SKB_GSO_ECN was set in RX path from patch bf296b125b21b8d558ceb6ec30bb4eba2730cd6b in tcp_gro_complete(). In patch 4e4f7cefb130af6aba6a393b2d13930b49390df9 (part of our first AccECN preparation patch series), it was changed into ACCECN to avoid corrupting CWR flag elsewhere.
And you are right that SKB_GSO_ECN is still been used for virtio and some drivers (drivers/net/ethernet/mellanox/mlx5/core/en_rx.c, ./drivers/net/ethernet/hisilicon/hns3/hns3_enet.c). Therefore, we plan to replace SKB_GSO_ECN with SKB_GSO_ACCECN in the two upcoming patches following this patch series. Following discussions with virtio-spec colleague (Parav Pandit in cc), it is suggested to fix this text first here before changing virtio-spec.
To clarify it, I would propose the following texts in next version for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN:
/* 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. */
/* 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 the CWR flag is not cleared by * any RFC3168 ECN offload, and thus keeping AccECN signal of TCP segments. */
Chia-Yu