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 19:15:24 -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 6:24 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 4 Oct 2022 18:02:56 -0700 Stanislav Fomichev wrote:
> > +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?
>
> I'd think so, worst case the driver can put xdp_md into a struct
> and container_of() to get to its own stack with whatever fields
> it needs.

Ack, seems like something worth exploring then.

The only issue I see with that is that we'd probably have to extend
the loading api to pass target xdp device so we can pre-generate
per-device bytecode for those kfuncs? And this potentially will block
attaching the same program to different drivers/devices?
Or, Martin, did you maybe have something better in mind?

> > 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?
>
> If there is a program on Tx then it'd be trivial - just do the
> info <-> descriptor translation in the opposite direction than Rx.
> TBH I'm not sure how it'd be done in the current approach, either.

Yeah, I don't think it magically works in any case. I'm just trying to
understand whether it's something we care to support out of the box or
can punt to the bpf programs themselves and say "if you care about
forwarding metadata, somehow agree on the format yourself".

> Now I questioned the BTF way and mentioned the Tx-side program in
> a single thread, I better stop talking...

Forget about btf, hail to the new king - kfunc :-D