Subject: Re: [PATCH bpf-next v3 1/5] netlink: specs: Add XDP RX checksum capability to XDP metadata specs

From: Jakub Kicinski <[email-masked]>
Date: Thu, 19 Feb 2026 09:13:44 -0800

On Thu, 19 Feb 2026 12:04:56 +0100 Lorenzo Bianconi wrote:
> > On Tue, 17 Feb 2026 09:33:56 +0100 Lorenzo Bianconi wrote:  
> > > + * In case of success, ``ip_summed`` is set to the RX checksum result. Possible
> > > + * values are:
> > > + * ``XDP_CHECKSUM_NONE``
> > > + * ``XDP_CHECKSUM_UNNECESSARY``
> > > + * ``XDP_CHECKSUM_COMPLETE``
> > > + *
> > > + * In case of success, ``cksum_meta`` contains the hw computed checksum value
> > > + * for ``XDP_CHECKSUM_COMPLETE`` or the ``csum_level`` for
> > > + * ``XDP_CHECKSUM_UNNECESSARY``. It is set to 0 for ``XDP_CHECKSUM_NONE``  
> > 
> > It's fairly common for NICs to report both csum complete and
> > unnecessary. Which one should the driver return in that case?  
> 
> Do you mean what is value for cksum_meta if we do not report csum_level for
> XDP_CHECKSUM_UNNECESSARY/CHECKSUM_UNNECESSARY use-case? (as suggested by
> Stanislav).

More fundamentally whether the API is right.

> My original idea is:
> - if the hw reports CHECKSUM_COMPLETE:
>   - ip_summed = XDP_CHECKSUM_COMPLETE
>   - cksum_meta contains the checksum computed by the hw
> - if the hw reports CHECKSUM_UNNECESSARY
>   - ip_summed = XDP_CHECKSUM_UNNECESSARY
>   - cksum_meta = csum_level <-- Stanislav suggests to drop this one
> - if the hw reports CHECKSUM_NONE
>   - ip_summed = XDP_CHECKSUM_NONE
>   - cksum_meta = 0

Off the top of my head drivers prefer reporting UNNECESSARY when they
have both, and reserve COMPLETE for cases where L4 could not be found
or is incorrect. Why don't we report both? We're using 3 args, we still
have 3 to go. We could turn ip_summed into a bitmap and have explicit
output args for both level and csum complete value?

One more thing I'd like us to at least have a plan for at this stage
is how to deal with COMPLETE + modified packet + XDP_PASS.
Right now some drivers discard COMPLETE when XDP is attached since
they can't be sure if XDP modifies the packet. Other drivers don't
and we end up with bad csum splat. Do we have a recommendation on
the correct behavior? If not - should we have a kfunc to adjust /
discard csum complete explicitly?