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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> 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?

Ack, thx for the explanation. Just for sake of understanding, is there
any NIC capable of reporting both csum_value and csum for the same packet
in the DMA descriptor? Or is this change needed to be future-proof?

> 
> 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?

At the moment there is no way to store the csum value we got running
bpf_xdp_metadata_rx_checksum() in order to be consumed during
xdp_buff/xdp_frame to skb conversion (this info can just be consumed in the
ebpf program bound to the NIC) but I guess the issue you pointed out can be
solved in the verifier during program load time. What do you think?

Regards,
Lorenzo


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