netdev - Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF

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Date:   Tue, 4 Oct 2022 18:02:56 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        brouer@...hat.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
        xdp-hints@...-project.net, larysa.zaremba@...el.com,
        memxor@...il.com, Lorenzo Bianconi <lorenzo@...nel.org>,
        mtahhan@...hat.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        dave@...cker.co.uk, Magnus Karlsson <magnus.karlsson@...el.com>,
        bjorn@...nel.org
Subject: Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW
 offload hints via BTF

On Tue, Oct 4, 2022 at 5:59 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 4 Oct 2022 17:25:51 -0700 Martin KaFai Lau wrote:
> > A intentionally wild question, what does it take for the driver to return the
> > hints.  Is the rx_desc and rx_queue enough?  When the xdp prog is calling a
> > kfunc/bpf-helper, like 'hwtstamp = bpf_xdp_get_hwtstamp()', can the driver
> > replace it with some inline bpf code (like how the inline code is generated for
> > the map_lookup helper).  The xdp prog can then store the hwstamp in the meta
> > area in any layout it wants.
>
> Since you mentioned it... FWIW that was always my preference rather than
> the BTF magic :)  The jited image would have to be per-driver like we
> do for BPF offload but that's easy to do from the technical
> perspective (I doubt many deployments bind the same prog to multiple
> HW devices)..

+1, sounds like a good alternative (got your reply while typing)
I'm not too versed in the rx_desc/rx_queue area, but seems like worst
case that bpf_xdp_get_hwtstamp can probably receive a xdp_md ctx and
parse it out from the pre-populated metadata?

Btw, do we also need to think about the redirect case? What happens
when I redirect one frame from a device A with one metadata format to
a device B with another?