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Date:   Tue, 18 Sep 2018 09:52:26 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     John Stultz <john.stultz@linaro.org>
cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case
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On Mon, 17 Sep 2018, John Stultz wrote:
> On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@kernel.org> wrote:
> > Also, I'm not entirely convinced that this "last" thing is needed at
> > all.  John, what's the scenario under which we need it?
> 
> So my memory is probably a bit foggy, but I recall that as we
> accelerated gettimeofday, we found that even on systems that claimed
> to have synced TSCs, they were actually just slightly out of sync.
> Enough that right after cycles_last had been updated, a read on
> another cpu could come in just behind cycles_last, resulting in a
> negative interval causing lots of havoc.
> 
> So the sanity check is needed to avoid that case.

Your memory serves you right. That's indeed observable on CPUs which
lack TSC_ADJUST.

@Andy: Welcome to the wonderful world of TSC.

Thanks,

	tglx