RE: [PATCH v10 1/2] virtio-net: Fix ECN feature descriptions

> -----Original Message-----
> From: Parav Pandit <[email protected]> 
> Sent: Friday, October 3, 2025 12:41 PM
> To: Chia-Yu Chang (Nokia) <[email protected]>; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; Koen De Schepper (Nokia) <[email protected]>; [email protected]; [email protected]; [email protected]; [email protected]
> Subject: RE: [PATCH v10 1/2] virtio-net: Fix ECN feature descriptions
> 
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> 
> > From: Chia-Yu Chang (Nokia) <[email protected]>
> > Sent: 01 October 2025 05:56 PM
> >
> > > -----Original Message-----
> > > From: Parav Pandit <[email protected]>
> > > Sent: Wednesday, October 1, 2025 12:51 PM
> > > To: Chia-Yu Chang (Nokia) <[email protected]>;
> > > [email protected]; [email protected]; [email protected]; 
> > > [email protected]; [email protected];
> > [email protected];
> > > [email protected]; [email protected]; [email protected]; Koen De 
> > > Schepper (Nokia) <[email protected]>;
> > > [email protected]; [email protected]; 
> > > [email protected]; [email protected]
> > > Subject: RE: [PATCH v10 1/2] virtio-net: Fix ECN feature 
> > > descriptions
> > >
> > >
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> > > > From: Chia-Yu Chang (Nokia) <[email protected]>
> > > > Sent: 01 October 2025 01:04 PM
> > > >
> > > > > -----Original Message-----
> > > > > From: Parav Pandit <[email protected]>
> > > > > Sent: Wednesday, October 1, 2025 6:24 AM
> > > > > To: Chia-Yu Chang (Nokia) <[email protected]>;
> > > > > [email protected]; [email protected]; 
> > > > > [email protected]; [email protected]; [email protected];
> > > > [email protected];
> > > > > [email protected]; [email protected]; [email protected]; Koen 
> > > > > De Schepper (Nokia) <[email protected]>;
> > > > > [email protected]; [email protected]; 
> > > > > [email protected]; [email protected]
> > > > > Subject: RE: [PATCH v10 1/2] virtio-net: Fix ECN feature 
> > > > > descriptions
> > > > >
> > > > >
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> > > > > > From: Chia-Yu Chang (Nokia) 
> > > > > > <[email protected]>
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: Parav Pandit <[email protected]>
> > > > > > > Sent: Tuesday, September 30, 2025 6:15 AM
> > > > > > > To: Chia-Yu Chang (Nokia) 
> > > > > > > <[email protected]>;
> > > > > > > [email protected]; [email protected]; 
> > > > > > > [email protected]; [email protected];
> > [email protected];
> > > > > > [email protected];
> > > > > > > [email protected]; [email protected]; [email protected]; 
> > > > > > > Koen De Schepper (Nokia) 
> > > > > > > <[email protected]>;
> > > > > > > [email protected]; [email protected]; 
> > > > > > > [email protected]; 
> > > > > > > [email protected]
> > > > > > > Subject: Re: [PATCH v10 1/2] virtio-net: Fix ECN feature 
> > > > > > > descriptions
> > > > > > >
> > > > > > >
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> > > > > > > On 29-09-2025 11:14 pm, Chia-Yu Chang (Nokia) wrote:
> > > > > > > >> -----Original Message-----
> > > > > > > >> From: Parav Pandit <[email protected]>
> > > > > > > >> Sent: Monday, September 29, 2025 9:48 AM
> > > > > > > >> To: Chia-Yu Chang (Nokia) 
> > > > > > > >> <[email protected]>;
> > > > > > > >> [email protected]; [email protected]; 
> > > > > > > >> [email protected]; [email protected]; 
> > > > > > > >> [email protected]; [email protected]; 
> > > > > > > >> [email protected];
> > > > [email protected];
> > > > > > > >> [email protected]; Koen De Schepper (Nokia) 
> > > > > > > >> <[email protected]>;
> > > > > > > >> [email protected]; [email protected]; 
> > > > > > > >> [email protected];
> > > > [email protected]
> > > > > > > >> Subject: RE: [PATCH v10 1/2] virtio-net: Fix ECN feature 
> > > > > > > >> descriptions
> > > > > > > >>
> > > > > > > >>
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> > > > > > > >>
> > [...]
> > > Virtio spec requirements are clearly written.
> > > So just by referring to RFC 3168 is as blurry as today in context of GSO.
> > >
> > > At best possibly it can be written as "device SHOULD do ECN and CWR 
> > > in first
> > pkt.. and subsequent packets SHOULD have cleared".
> > >
> > > If that is not aligning, than better to not make it more blurry.
> > > ACCECN feature bit can be more crisply defined in that case.
> >
> > OK, then could you let us know if we make below changes in the next 
> > version, are they clear enough for virtio spec?
> >
> >
> > 1. VIRTIO_NET_F_GUEST_ECN (9): Driver can receive TSO with the CWR 
> > flag set. This CWR flag is only applied to the first packet, and the 
> > CWR flag of subsequent packets should be cleared.
> >
> > 2. VIRTIO_NET_F_HOST_ECN (13): Device can receive TSO with the CWR 
> > flag set. This CWR flag is only applied to the first packet, and the 
> > CWR flag of subsequent packets should be cleared.
> >
> > 3. If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the 
> > VIRTIO_NET_HDR_GSO_ECN bit in gso_type indicates that the CWR flag of 
> > the TCP packet applied only to the first packet and is cleared in 
> > subsequent packets.
> >
> > 4. The driver SHOULD NOT send to the device TCP packets requiring 
> > segmentation offload with the CWR flag set ONLY for to the first 
> > packet and cleared in subsequent packets, unless the 
> > VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the driver 
> > MUST set the VIRTIO_NET_HDR_GSO_ECN bit in gso_type.
> >
> > 5. The device SHOULD NOT send to the driver TCP packets requiring 
> > segmentation offload with the CWR flag set ONLY for to the first 
> > packet and cleared in subsequent packets, unless the 
> > VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the driver 
> > MUST set the VIRTIO_NET_HDR_GSO_ECN bit in gso_type.
> >
> 
> Also, while it is good to document existing behaviour of the spec, devices and drivers, I believe that ACCECN need not have to wait for the documentation of existing GSO_ECN.
> 
> However, I am bit puzzled with the Linux kernel commit [1] of yours that hints that ACCECN may not need any special handling.
> 
> You described,
> 
>     If NIC is completely unaware of RFC3168 ECN (doesn't support
>     NETIF_F_TSO_ECN) or its TSO engine can be set to not touch CWR flag
>     despite supporting also NETIF_F_TSO_ECN, TSO could be safely used
>     with AccECN on such NIC. This should be evaluated per NIC basis
>     (not done in this patch series for any NICs).
> 
> Based on above commit log, can we say that when existing GSO_ECN is not negotiated, ACCECN can work as_is without the patch_2?
> 
> [1] commit 023af5a72ab16 ("gso: AccECN support")

Indeed, this patch description implies that NIC that currently don't support TCP_ECN will naturally support ACCECN.
But an extra TSO_ACCECN flag will make this into an explicit requirement, like our patch in the Linux.

Related to another email about existing driver, one solution would be replace SKB_GSO_TCP_ECN with SKB_GSO_TCP_ACCECN, like in 4e4f7cefb130af6aba6a393b2d13930b49390df9.
This can avoid CWR corruption somewhere after GRO, and we plan to submit related patches (old driver, virtio-related) into net-next after this is approved in virtio-spec.