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Message-Id: <B8C05946-43BB-40A4-A564-53904FAF8CC0@amacapital.net>
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.
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