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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 16 Jan 2020 12:19:50 -0800
Message-ID: <CALCETrX9+PZ1h6xex2WZcSqNT7W-6R-E95jv9hLhSdAzhMCrTA@mail.gmail.com>
Subject: Re: [RFC PATCH v4 08/11] lib: vdso: allow fixed clock mode
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, nathanl@linux.ibm.com,
        Arnd Bergmann <arnd@arndb.de>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Andrew Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        "open list:MIPS" <linux-mips@vger.kernel.org>,
        X86 ML <x86@kernel.org>
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On Thu, Jan 16, 2020 at 12:14 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>
> Can you please adjust the prefix for future patches to lib/vdso: and
> start the sentence after the colon with an uppercase letter?
>
> > On arches like POWERPC, the clock is always the timebase, it
>
> Please spell out architectures. Changelogs are not space constraint.
>
> > cannot be changed on the fly and it is always VDSO capable.
>
> Also this sentence does not make sense as it might suggests that
> architectures with a fixed compile time known clocksource have something
> named timebase. Something like this is more clear:
>
> Some architectures have a fixed clocksource which is known at compile
> time and cannot be replaced or disabled at runtime, e.g. timebase on
> PowerPC. For such cases the clock mode check in the VDSO code is
> pointless.
>

I wonder if we should use this on x86 bare-metal if we have
sufficiently invariant TSC.  (Via static_cpu_has(), not compiled in.)
Maybe there is no such x86 machine.

I really really want Intel or AMD to introduce machines where the TSC
pinky-swears to count in actual nanoseconds.

--Andy