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Date:   Sat, 6 Oct 2018 17:27:33 -0300
From:   Marcelo Tosatti <mtosatti@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Matt Rickard <matt@softrans.com.au>,
        Stephen Boyd <sboyd@kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Florian Weimer <fweimer@redhat.com>,
        KY Srinivasan <kys@microsoft.com>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI
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On Thu, Oct 04, 2018 at 03:15:32PM -0700, Andy Lutomirski wrote:
> For better or for worse, I'm trying to understand this code.  So far,
> I've come up with this patch:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso-tglx&id=14fd71e12b1c4492a06f368f75041f263e6862bf
> 
> Is it correct, or am I missing some subtlety?

The master clock, when initialized, has a pair

masterclockvalues=(TSC value, time-of-day data).

When updating the guest clock, we only update relative to (TSC value)
that was read on masterclock initialization.

See the following comment on x86.c:

/*
 *
 * Assuming a stable TSC across physical CPUS, and a stable TSC
 * across virtual CPUs, the following condition is possible.
 * Each numbered line represents an event visible to both
 * CPUs at the next numbered event.
...

When updating the "masterclockvalues" pair, all vcpus are 
stopped.