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

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Message-ID: <533129a4-7f4e-e7a6-407c-f15b6acbb0e2@nvidia.com>
Date:   Fri, 22 Oct 2021 19:56:06 +0300
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "Lorenz Bauer" <lmb@...udflare.com>
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>,
        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 2021-10-20 19:16, Toke Høiland-Jørgensen wrote:
> Lorenz Bauer <lmb@...udflare.com> writes:
> 
>>> +bool cookie_init_timestamp_raw(struct tcphdr *th, __be32 *tsval, __be32 *tsecr)
>>
>> I'm probably missing context, Is there something in this function that
>> means you can't implement it in BPF?
> 
> I was about to reply with some other comments but upon closer inspection
> I ended up at the same conclusion: this helper doesn't seem to be needed
> at all?

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.