linux-kernel - Re: [3/8] x86/tsc: Store and check TSC ADJUST MSR

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Date:   Mon, 30 Jan 2017 11:20:25 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Henning Schild <henning.schild@...mens.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Borislav Petkov <bp@...en8.de>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [3/8] x86/tsc: Store and check TSC ADJUST MSR

Henning,

On Fri, 27 Jan 2017, Henning Schild wrote:
> 
> did you by any chance look into TSC synchronization by adjusting the
> absolute value (MSR_IA32_TSC) as well? As far as i have seen Linux did
> that a long time ago and eventually it was stopped because it caused more
> harm than good.

I was involved in both developing the TSC sync patches and ripping them out
again.

The problem with writing TSC directly is that you really _CANNOT_ reliably
handle run-time differences and SMI/NMI induced deltas. With the adjust MRS
it's a halfways sane thing to do, except for the brokeness of that botch
job vs. the TSC deadline timer.

> The ADJUST MSR offers an easy way to synchronize, still taking care of
> all the special cases resulted in an 8-patch series. Synching without
> that using the absolute value is likely much harder, but that series
> might be a good foundation.

I'm not even thinking about bringing the pure TSC based sync back.

> The big question is whether we can rely on all future CPUs to 
> support that MSR. Do "new MSRs" disappear again at some point? If we
> can not rely on the ADJUST MSR, now might be a good time to revisit the
> idea of synching the absolute values.

There is nothing you can ever be sure about, but I doubt that the ADJUST
MSR is going to vanish.

> I remember having read somewhere that this series might get backported
> to longterm kernels, what is the status on that?

No idea.

Thanks,

	tglx