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

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Date:   Thu, 4 Oct 2018 07:00:45 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        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 Oct 4, 2018, at 1:11 AM, Peter Zijlstra <peterz@...radead.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.

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