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

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Message-ID:
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Date: Wed, 19 Nov 2025 11:22:08 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Paolo Abeni <pabeni@...hat.com>, "edumazet@...gle.com"
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Subject: RE: [PATCH v6 net-next 03/14] net: update commnets for
 SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN

> -----Original Message-----
> From: Paolo Abeni <pabeni@...hat.com> 
> Sent: Wednesday, November 19, 2025 11:40 AM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>; edumazet@...gle.com; parav@...dia.com; linux-doc@...r.kernel.org; corbet@....net; horms@...nel.org; dsahern@...nel.org; kuniyu@...gle.com; bpf@...r.kernel.org; netdev@...r.kernel.org; dave.taht@...il.com; jhs@...atatu.com; kuba@...nel.org; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; andrew+netdev@...n.ch; donald.hunter@...il.com; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; ncardwell@...gle.com; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire <cheshire@...le.com>; rs.ietf@....at; Jason_Livingood@...cast.com; Vidhi Goel <vidhi_goel@...le.com>
> Subject: Re: [PATCH v6 net-next 03/14] net: update commnets for SKB_GSO_TCP_ECN and SKB_GSO_TCP_ACCECN
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> 
> 
> 
> On 11/19/25 11:24 AM, Chia-Yu Chang (Nokia) wrote:
> > I was thinking to totally remove ECN from Rx path,
> 
> ??? do you mean you intend to remove the existing virtio_net ECN support? I guess/hope I misread the above.
> 
> Note that removing features from virtio_net is an extreme pain at best, and more probably simply impossible - see the UFO removal history.
> 
> Please clarify, thanks!
> 
> Paolo

This ECN flag on RX path shall not be used in Rx path for forwarding scenario. But it can still be used on Tx path in virtio_net.

And on RX path, new ACCECN flag shall be used to avoid breaking CWR flag for latter GSO Tx in forwarding scenario.

Let me borrow an example from Ilpo:

SKB_GSO_TCP_ECN will not replicate the same TCP header flags in a forwarding scenario:
Segment 1 CWR set
Segment 2 CWR set

GRO rx and GSO tx with SKB_GSO_TCP_ECN, after forwarding outputs these segments:
Segment 1 CWR set
Segment 2 CWR cleared

Thus, the ACE field in Segment 2 no longer contains the same value as it was sent with.


So, maybe a table below better represent this?
+===============+======================+===========================+
|               |   SKB_GSO_TCP_ECN    |     SKB_GSO_TCP_ACCECN    |
+===============+======================+===========================+
|               | The 1st TCP segment  |   The TCP segment uses    |
|    Tx path    | has CWR set and      |  the CWR flag as part of  |
|               | suqsequent segments  |  ACE signal, and the CWR  |
|               | have CWR cleared.    |   flag is not modified.   |
+---------------+----------------------+---------------------------+
|    Rx path    | Shall not be used to |  Used to indicate latter  |
| of forwarding | avoid potential ACE  |  GSO Tx NOT to clear CWR  |
|    scenario   |  signal corruption.  | flag from the 2nd segment |
+===============+======================+===========================+


Chia-Yu