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 18:24:51 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Stanislav Fomichev <sdf@...gle.com>
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, 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.

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