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:
 <PAXPR07MB79842DF3D2028BB3366F0AF6A3D7A@PAXPR07MB7984.eurprd07.prod.outlook.com>
Date: Wed, 19 Nov 2025 10:24:33 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Paolo Abeni <pabeni@...hat.com>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "parav@...dia.com" <parav@...dia.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: Tuesday, November 18, 2025 1:02 PM
> 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.
> 
> 
> 
> Note: typo in the subj
> 
> On 11/14/25 8:13 AM, chia-yu.chang@...ia-bell-labs.com wrote:
> > From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> >
> > No functional changes.
> 
> Some real commit message is needed.
> 
> >
> > 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.
> 
> Stating both that is used by virtio_net and can not be used in the RX path is a bit confusing. Random Contributor may be tempted from removing ECN support from virtio_net
> 
> Please state explicitly:
> - why it makes sense to use this in virtio_net
> - this must not be used in the RX path _outside_ the virtio net driver
> 
> something alike:
> 
> /* For Tx, this indicates the first TCP segment has CWR set, and any
>  * subsequent segment in the same skb has CWR cleared. 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 outside the virtio_net driver. Such exception exist due to
>  * <reason>
>  */
> 
> /P
Hi Paolo and Ilpo,

I was thinking to totally remove ECN from Rx path, and add the comments only in AccECN, like below:
Because we could use SKB_GSO_TCP_ACCECN in Rx to explicitly tell latter GSO Tx in a forwarding case that do NOT clean CWR flag.
What do you think?


/* For Tx, this indicates the first TCP segment has CWR set, and any
 * subsequent segment in the same skb has CWR cleared. 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
 * ACE signal of AccECN segments. Therefore, this cannot be used on Rx.
 */
SKB_GSO_TCP_ECN = 1 << 2,
[...]
/* For TX, this indicates the TCP segment uses the CWR flag as part of
* ACE signal, and the CWR flag is not modified in the skb. 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 ACE
* signal of AccECN segments. This is particularly used for Rx of
* virtio_net driver in order to tell latter GSO Tx in a forwarding
* scenario that it is NOT ok to clean CWR flag from the 2nd segment.
 */
SKB_GSO_TCP_ACCECN = 1 << 19,

Chia-Yu