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Date:   Thu, 4 Oct 2018 21:31:50 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        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 07:00:45AM -0700, Andy Lutomirski wrote:
> > On Oct 4, 2018, at 1:11 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> >> On Thu, Oct 04, 2018 at 09:54:45AM +0200, Vitaly Kuznetsov wrote:
> >> I was hoping to hear this from you :-) If I am to suggest how we can
> >> move forward I'd propose:
> >> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling
> >> is supported).
> >> - Check if non-masterclock mode is still needed. E.g. HyperV's TSC page
> >> clocksource is a single page for the whole VM, not a per-cpu thing. Can
> >> we think that all the buggy hardware is already gone?
> > 
> > No, and it is not the hardware you have to worry about (mostly), it is
> > the frigging PoS firmware people put on it.
> > 
> > Ever since Nehalem TSC is stable (unless you get to >4 socket systems,
> > after which it still can be, but bets are off). But even relatively
> > recent systems fail the TSC sync test because firmware messes it up by
> > writing to either MSR_TSC or MSR_TSC_ADJUST.
> > 
> > But the thing is, if the TSC is not synced, you cannot use it for
> > timekeeping, full stop. So having a single page is fine, it either
> > contains a mult/shift that is valid, or it indicates TSC is messed up
> > and you fall back to something else.
> > 
> > There is no inbetween there.
> > 
> > For sched_clock we can still use the global page, because the rate will
> > still be the same for each cpu, it's just offset between CPUs and the
> > code compensates for that.
> 
> But if we’re in a KVM guest, then the clock will jump around on the
> same *vCPU* when the vCPU migrates.

Urgh yes..

> But I don’t see how kvmclock helps here, since I don’t think it’s used
> for sched_clock.

I get hits on kvm_sched_clock, but haven't looked further.

Anyway, Most of the argument still holds, either TSC is synced or it is
not and it _really_ should not be used. Not much middle ground there.