Re: [RFC PATCH bpf-next v1 0/7] xdp: RX checksum metadata hint and checksum assertion over redirect

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> Hi Lorenzo,

Hi Vladimir,

> 
> Sorry -- I blindly missed your earlier RX-checksum series before I posted
> mine, thanks Stanislav for the pointer.

No worries. My series is just adding a new kfunc to report the rx-csum result
to an ebpf program running on the NIC. It does not take into account the
XDP_REDIRECT use-case. This is orthogonal and we can work on it later.

> 
> To answer your question: yes, I'm happy to take on the driver selftest
> Jakub asked for.

Cool, thx :)

> 
> As for my own series, the read side clearly overlaps yours and you own it,
> so I'll drop my bpf_xdp_metadata_rx_csum() hint and its driver bits.
> 
> What's left that is genuinely separate is the "assertion" half -- a non-dev-bound
> bpf_xdp_assert_rx_csum() that preserves the HW verdict across a
> cpumap/redirect: it sets a flag on the xdp_buff that rides into the
> xdp_frame and becomes skb->ip_summed = CHECKSUM_UNNECESSARY in
> __xdp_build_skb_from_frame().
> 
> Should I resend that as a small standalone series (v2, assert-only)?
> It also looks like a PoC that you and Jakub discussed on v3 [1].

I think we should address the 'CHECKSUM_COMPLETE' use case for it, let's work
on the problem later, even Jesper is interested in it ;)

> 
> A few things I'd like to confirm before writing the test:
> 
> 1. API for v4. In the v3 discussion you agreed to rework the API to report
>    both COMPLETE and UNNECESSARY (+ csum_level), per Jakub. Do you plan to
>    send that in v4, or should the driver selftest target the current v3
>    signature (enum xdp_checksum + cksum_meta)? I'd rather write the test
>    against the API you intend to keep.

IIRC I addressed the request (the code is in [0]). What is missing is just the
self-test.

> 
> 2. Documented behavior. The selftest is meant to "check the documented
>    expectation", so which rule should it assert -- "a driver must never
>    report CHECKSUM_COMPLETE while an XDP program is attached", or that the
>    driver downgrades/repairs COMPLETE on the XDP_PASS path? I'll write the
>    doc paragraph and the test to match whatever we settle on.

ack

> 
> 3. Drivers. Your series adds veth and ice; I don't see mlx5e -- was that
>    intentional (left to the driver maintainers)? I had an mlx5e
>    implementation in my v1 and I'm happy to contribute it to your series if
>    it's useful.

My series does not intend to address all the drivers :)

> 
> For the test itself I was thinking of extending
> tools/testing/selftests/drivers/net/hw/xdp_metadata.py, gated on the new
> "checksum" xdp-rx-metadata feature, with good-csum / bad-csum / modify +
> XDP_PASS cases. Does that match what you and Jakub had in mind?

That was my idea too.

Regards,
Lorenzo

> 
> [1] https://lore.kernel.org/bpf/20260217-bpf-xdp-meta-rxcksum-v3-0-30024c50ba71@kernel.org/
> 
> Thanks,
> Vladimir

[0] https://github.com/LorenzoBianconi/net-next/commits/b4/bpf-xdp-meta-rxcksum/

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