linux-kernel - Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC

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Message-ID: <20180314131237.GL4064@hirez.programming.kicks-ass.net>
Date:   Wed, 14 Mar 2018 14:12:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jason Vas Dias <jason.vas.dias@...il.com>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        andi <andi@...stfloor.org>
Subject: Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle
 CLOCK_MONOTONIC_RAW

On Wed, Mar 14, 2018 at 12:55:20PM +0000, Jason Vas Dias wrote:
> > While CPU_CLOCK is TSC based, there is no guarantee it has any
> > correlation to CLOCK_MONOTONIC_RAW (even if that is also TSC based).
> >
> > (although, I think I might have fixed that recently and it might just
> > work, but it's very much not guaranteed).
> 
> Yes, I believe the CPU_CLOCK is effectively the converted TSC -
> it does appear to correlate well with the new CLOCK_MONOTONIC_RAW
> values from the patched VDSO.

It (now) runs at the same rate, but there is no guarantee for this, in
fact it didn't for a very long time.

Relying on this is broken.