linux-kernel - Re: [PATCH v3] x86/tsc: add X86_FEATURE_TSC_KNOWN

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Message-ID: <alpine.DEB.2.20.1610211005130.4797@nanos>
Date:   Fri, 21 Oct 2016 10:05:42 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Bin Gao <bin.gao@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        John Stultz <john.stultz@...aro.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, bin.gao@...el.com
Subject: Re: [PATCH v3] x86/tsc: add X86_FEATURE_TSC_KNOWN_FREQ flag

On Fri, 21 Oct 2016, Peter Zijlstra wrote:
> On Thu, Oct 20, 2016 at 09:37:50PM +0200, Thomas Gleixner wrote:
> 
> > Well, we have the same issue on other platforms/models which set the
> > reliable flag.
> 
> I was not aware we had other platforms doing this, git grep tells me
> intel-mid does this as well..
> 
> > So one sanity check we can do is to read the IA32_TSC_ADJUST MSR on all
> > cores. They should all have the same value (usually 0) or at least have a
> > very minimal delta. If that's off by more than 1us then something is fishy
> > especially on single socket systems. We could at least WARN about it.
> > 
> > We could do this in idle occasionally as well, so we can detect the dreaded
> > "SMI wants to hide the cycles" crapola.
> 
> Indeed, that sounds like the best we can; and probably should; do.

I'll have a look at that in the next days.

Thanks,

	tglx