netdev - Re: [PATCH bpf-next] bpf: Add BPF-helper for accessing CLOCK

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Date:   Mon, 6 Jun 2022 08:57:25 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Joanne Koong <joannelkoong@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Geliang Tang <geliang.tang@...e.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH bpf-next] bpf: Add BPF-helper for accessing CLOCK_TAI

On Mon, Jun 6, 2022 at 3:38 AM Kurt Kanzenbach <kurt@...utronix.de> wrote:
>
> From: Jesper Dangaard Brouer <brouer@...hat.com>
>
> Commit 3dc6ffae2da2 ("timekeeping: Introduce fast accessor to clock tai")
> introduced a fast and NMI-safe accessor for CLOCK_TAI. Especially in time
> sensitive networks (TSN), where all nodes are synchronized by Precision Time
> Protocol (PTP), it's helpful to have the possibility to generate timestamps
> based on CLOCK_TAI instead of CLOCK_MONOTONIC. With a BPF helper for TAI in
> place, it becomes very convenient to correlate activity across different
> machines in the network.

That's a fresh feature. It feels risky to bake it into uapi already.
imo it would be better to annotate tk_core variable in vmlinux BTF.
Then progs will be able to read all possible timekeeper offsets.