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Date:   Tue, 18 Sep 2018 12:41:57 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     John Stultz <john.stultz@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.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 Tue, 18 Sep 2018, Thomas Gleixner wrote:
> On Tue, 18 Sep 2018, Thomas Gleixner wrote:
> > On Tue, 18 Sep 2018, Peter Zijlstra wrote:
> > > > Your memory serves you right. That's indeed observable on CPUs which
> > > > lack TSC_ADJUST.
> > > 
> > > But, if the gtod code can observe this, then why doesn't the code that
> > > checks the sync?
> > 
> > Because it depends where the involved CPUs are in the topology. The sync
> > code might just run on the same package an simply not see it. Yes, w/o
> > TSC_ADJUST the TSC sync code can just fail to see the havoc.
> 
> Even with TSC adjust the TSC can be slightly off by design on multi-socket
> systems.

Here are the gory details:

   https://lore.kernel.org/lkml/3c1737210708230408i7a8049a9m5db49e6c4d89ab62@mail.gmail.com/

The changelog has an explanation as well.

    d8bb6f4c1670 ("x86: tsc prevent time going backwards")

I still have one of the machines which is affected by this.

Thanks,

	tglx