linux-kernel - Re: [PATCH RFC] x86/tsc: Make recalibration default on for TSC_KNOWN

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Date:   Mon, 22 May 2023 10:14:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, paulmck@...nel.org,
        rui.zhang@...el.com, x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Feng Tang <feng.tang@...el.com>
Subject: Re: [PATCH RFC] x86/tsc: Make recalibration default on for
 TSC_KNOWN_FREQ cases

On Mon, May 22 2023 at 11:30, Feng Tang wrote:
> Commit a7ec817d5542 ("x86/tsc: Add option to force frequency
> recalibration with HW timer") was added to handle cases that the
> firmware has bug and provides a wrong TSC frequency number, and it
> is optional given that this kind of firmware issue rarely happens
> (Paul reported once [1]).
>
> But Rui reported that some Sapphire Rapids platform met this issue
> again recently, and as firmware is also a kind of 'software' which
> can't be bug free, make the recalibration default on. When the
> values from firmware and HW timer's calibration have big gap,
> raise a warning and let vendor to check which side is broken.

Sure firmware can have bugs, but if firmware validation does not even
catch such a trivially to detect bug, then their validation is nothing
else than rubber stamping. Seriously.

Are any of these affected platforms shipping already or is this just
Intel internal muck?

> One downside is, many VMs also has X86_FEATURE_TSC_KNOWN_FREQ set,
> and they will also do this recalibration.

It's also pointless for those SoCs which lack legacy hardware.

So why do you force this on everyone?

Thanks,

        tglx