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

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Date:   Tue, 07 Jun 2022 11:14:12 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH bpf-next] bpf: Add BPF-helper for accessing CLOCK_TAI

Alexei,

On Mon, Jun 06 2022 at 08:57, Alexei Starovoitov wrote:
> 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.

What? That's just support for a different CLOCK. What's so risky about
it?

> imo it would be better to annotate tk_core variable in vmlinux BTF.
> Then progs will be able to read all possible timekeeper offsets.

We are exposing APIs. APIs can be supported, but exposing implementation
details creates ABIs of the worst sort because that prevents the kernel
from changing the implementation. We've seen the fallout with the recent
tracepoint changes already.

Thanks,

        tglx