linux-kernel - Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK

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Message-ID: <20181006202731.GC7129@amt.cnet>
Date:   Sat, 6 Oct 2018 17:27:33 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Florian Weimer <fweimer@...hat.com>,
        KY Srinivasan <kys@...rosoft.com>,
        devel@...uxdriverproject.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI
 support

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.