netdev - Re: [PATCH bpf-next 09/10] bpf: Add a helper to issue timestamp cookies in XDP

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Date:   Wed, 27 Oct 2021 09:34:31 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Maxim Mikityanskiy <maximmi@...dia.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        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>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Joe Stringer <joe@...ium.io>, Tariq Toukan <tariqt@...dia.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH bpf-next 09/10] bpf: Add a helper to issue timestamp
 cookies in XDP

On Fri, 22 Oct 2021 at 17:56, Maxim Mikityanskiy <maximmi@...dia.com> wrote:
>
> tcp_time_stamp_raw() uses ktime_get_ns(), while bpf_ktime_get_ns() uses
> ktime_get_mono_fast_ns(). Is it fine to use ktime_get_mono_fast_ns()
> instead of ktime_get_ns()? I'm a bit worried about this note in
> Documentation/core-api/timekeeping.rst:
>
>  > most drivers should never call them,
>  > since the time is allowed to jump under certain conditions.

That depends on what happens when the timestamp is "off". Since you're
sending this value over the network I doubt that the two methods will
show a difference.

Lorenz

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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