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

[Stanislav Fomichev <sdf.kernel@gmail.com>]

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/