linux-kernel - Re: [RFC][PATCH 3/4] arm64: vdso: Fix nsec handling for CLOCK_MONOTONIC

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Date:   Tue, 30 May 2017 10:38:43 +0100
From:   Will Deacon <will.deacon@....com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Daniel Mentz <danielmentz@...gle.com>
Subject: Re: [RFC][PATCH 3/4] arm64: vdso: Fix nsec handling for
 CLOCK_MONOTONIC_RAW

On Fri, May 26, 2017 at 08:33:54PM -0700, John Stultz wrote:
> From: Will Deacon <will.deacon@....com>
> 
> Commit 45a7905fc48f ("arm64: vdso: defer shifting of nanosecond
> component of timespec") fixed sub-ns inaccuracies in our vDSO
> clock_gettime implementation by deferring the right-shift of the
> nanoseconds components until after the timespec addition, which
> operates on left-shifted values. That worked nicely until
> support for CLOCK_MONOTONIC_RAW was added in 49eea433b326
> ("arm64: Add support for CLOCK_MONOTONIC_RAW in clock_gettime()
> vDSO"). Noticing that the core timekeeping code never set
> tkr_raw.xtime_nsec, the vDSO implementation didn't bother
> exposing it via the data page and instead took the unshifted
> tk->raw_time.tv_nsec value which was then immediately shifted
> left in the vDSO code.
> 
> Now that the core code is actually setting tkr_raw.xtime_nsec,
> we need to take that into account in the vDSO by adding it to
> the shifted raw_time value. Rather than do that at each use (and
> expand the data page in the process), instead perform the
> shift/addition operation when populating the data page and
> remove the shift from the vDSO code entirely.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Miroslav Lichvar <mlichvar@...hat.com>
> Cc: Richard Cochran <richardcochran@...il.com>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Stephen Boyd <stephen.boyd@...aro.org>
> Cc: Kevin Brodsky <kevin.brodsky@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Daniel Mentz <danielmentz@...gle.com>
> Reported-by: John Stultz <john.stultz@...aro.org>
> Acked-by: Acked-by: Kevin Brodsky <kevin.brodsky@....com>

I don't think Kevin liked it *that* much ^^

Will