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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 9 Oct 2018 13:09:42 -0700
Message-ID: <CALCETrW6b8=dU6vkXNS-rW1GPzJTbVxuVNsU4aoD_NwwobVQcg@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 Tue, Oct 9, 2018 at 8:28 AM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> On Mon, Oct 08, 2018 at 10:38:22AM -0700, Andy Lutomirski wrote:
> > On Mon, Oct 8, 2018 at 8:27 AM Marcelo Tosatti <mtosatti@redhat.com> wrote:

> > I read the comment three more times and even dug through the git
> > history.  It seems like what you're saying is that, under certain
> > conditions (which arguably would be bugs in the core Linux timing
> > code),
>
> I don't see that as a bug. Its just a side effect of reading two
> different clocks (one is CLOCK_MONOTONIC and the other is TSC),
> and using those two clocks to as a "base + offset".
>
> As the comment explains, if you do that, can't guarantee monotonicity.
>
> > actually calling ktime_get_boot_ns() could be non-monotonic
> > with respect to the kvmclock timing.  But get_kvmclock_ns() isn't used
> > for VM timing as such -- it's used for the IOCTL interfaces for
> > updating the time offset.  So can you explain how my patch is
> > incorrect?
>
> ktime_get_boot_ns() has frequency correction applied, while
> reading masterclock + TSC offset does not.
>
> So the clock reads differ.
>

Ah, okay, I finally think I see what's going on.  In the kvmclock data
exposed to the guest, tsc_shift and tsc_to_system_mul come from
tgt_tsc_khz, whereas master_kernel_ns and master_cycle_now come from
CLOCK_BOOTTIME.  So the kvmclock and kernel clock drift apart at a
rate given by the frequency shift and then suddenly agree again every
time the pvclock data is updated.

Is there a reason to do it this way?