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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 6 Oct 2018 15:28:05 -0700
Message-ID: <CALCETrWqze2mifOdFc0GJYHtHGKiKX2Zdi5Kz87OyUogbqD15w@mail.gmail.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        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>
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On Sat, Oct 6, 2018 at 1:29 PM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> 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.

I don't see the problem.  The masterclock data is updated here:

    host_tsc_clocksource = kvm_get_time_and_clockread(
                    &ka->master_kernel_ns,
                    &ka->master_cycle_now);

kvm_get_time_and_clockread() gets those values from
do_monotonic_boot(), which, barring bugs, should cause
get_kvmclock_ns() to return exactly the same thing as
ktime_get_boot_ns() + ka->kvmclock_offset, albeit in a rather
roundabout manner.

So what am I missing?  Is there actually something wrong with my patch?


>
> See the following comment on x86.c:

I read that comment, and it's not obvious to me how it's related.