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Date:   Thu, 27 Sep 2018 16:41:12 +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:
> > So if the TSC on CPU1 is slightly behind the TSC on CPU0 then now1 can be
> > smaller than cycle_last. The TSC sync stuff does not catch the small delta
> > for unknown raisins. I'll go and find that machine and test that again.
> 
> Of course it does not trigger anymore. We accumulated code between the
> point in timekeeping_advance() where the TSC is read and the update of the
> VDSO data.
> 
> I'll might have to get an 2.6ish kernel booted on that machine and try with
> that again. /me shudders

Actually it does happen, because the TSC is very slowly drifting apart due
to SMI wreckage trying to hide itself. It just takes a very long time.

Thanks,

	tglx