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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On Jun 30, Stanislav Fomichev wrote:
> On 06/30, Vladimir Vdovin wrote:
> > This series lets XDP programs work with the hardware RX checksum verdict:
> > read what the NIC concluded about a packet, and carry a "the L4 checksum
> > is correct" assertion across a redirect so the stack does not revalidate
> > it in software.
> > 
> > When an XDP program redirects a frame to a cpumap (or any other path that
> > rebuilds an skb from an xdp_frame via __xdp_build_skb_from_frame()), the
> > HW RX checksum status is lost and the stack revalidates the L4 checksum in
> > software.
> > 
> > Two kfuncs are added:
> > 
> >  - bpf_xdp_metadata_rx_csum(): a device-bound RX-metadata hint, like the
> >    existing rx_hash / rx_vlan_tag ones.  It reports enum xdp_csum_status
> >    (XDP_CSUM_NONE / XDP_CSUM_VERIFIED) and is implemented for mlx5e, ice
> >    and veth.
> > 
> >  - bpf_xdp_assert_rx_csum(): a generic, non-device-bound kfunc that lets
> >    the program assert the L4 checksum is correct.  It sets a buff flag
> >    that rides into the xdp_frame, and __xdp_build_skb_from_frame() turns
> >    it into skb->ip_summed = CHECKSUM_UNNECESSARY.  The kernel cannot
> >    verify the assertion; the program takes responsibility, as it already
> >    does when rewriting packet contents.
> > 
> > Posted as RFC to get feedback on:
> > 
> >  - whether the read hint (bpf_xdp_metadata_rx_csum() and its driver
> >    support) belongs in this series at all.  bpf_xdp_assert_rx_csum() is
> >    self-contained and already covers the main use case: a program that
> >    computes or fixes the L4 checksum itself, or trusts the source, and
> >    wants the rebuilt skb to skip software revalidation.  The read hint is
> >    an optimization for programs that did not touch the payload and only
> >    want to relay the hardware verdict.  These could just as well be two
> >    independent series (assert-only first);
> >  - the kfunc naming, bpf_xdp_assert_rx_csum() in particular.
> > 
> > Testing:
> > 
> >  - new selftest xdp_cpumap_rx_csum drives a frame through a native-XDP
> >    veth into a cpumap redirect and checks, via fexit on
> >    __xdp_build_skb_from_frame(), that the rebuilt skb is
> >    CHECKSUM_UNNECESSARY iff the program called bpf_xdp_assert_rx_csum();
> >  - xdp_metadata calls bpf_xdp_metadata_rx_csum() over veth and checks both
> >    verdicts: XDP_CSUM_NONE for an AF_XDP-injected frame and
> >    XDP_CSUM_VERIFIED for one sent through the stack.
> 
> This was posted somewhat recently from Lorenzo (and had a fair bit of
> discussion), but there haven't been a follow up:
> https://lore.kernel.org/bpf/20260217-bpf-xdp-meta-rxcksum-v3-0-30024c50ba71@kernel.org/

Hi Vladimir and Stanislav,

AFAIK in my series we are just missing the drv self-test requested by Jakub.
I have not the time to look into it yet.
@Vladimir: if you have any free-cycles, do you agree to introduce the missing
 test requested by Jakub to my series? Thanks in advance.

Regards,
Lorenzo

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