Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 18 Sep 2018 10:30:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
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
Message-ID: <20180918083055.GJ24106@hirez.programming.kicks-ass.net>
References: <20180914125006.349747096@linutronix.de>
 <20180914125118.909646643@linutronix.de>
 <CALCETrXh70oFHfT0Z-JXcnAumhAgLqSea5UiPY3WXu=v2GcKTg@mail.gmail.com>
 <CALAqxLUv9Rd_eA7u9gZNFvTNrT1Z67RpHLozdeNC0W2yZm=Heg@mail.gmail.com>
 <alpine.DEB.2.21.1809180948570.3558@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.21.1809180948570.3558@nanos.tec.linutronix.de>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Sep 18, 2018 at 09:52:26AM +0200, Thomas Gleixner wrote:
> 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.

But, if the gtod code can observe this, then why doesn't the code that
checks the sync?