linux-kernel - Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified platforms

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Date:   Wed, 01 Dec 2021 00:19:43 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org
Cc:     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>, x86@...nel.org,
        linux-kernel@...r.kernel.org, rui.zhang@...el.com,
        andi.kleen@...el.com, len.brown@...el.com, tim.c.chen@...el.com
Subject: Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for
 qualified platforms

On Tue, Nov 30 2021 at 14:48, Paul E. McKenney wrote:
> On Tue, Nov 30, 2021 at 10:55:45PM +0100, Thomas Gleixner wrote:
>> > OK, HPET or nothing, then.
>> 
>> Older machines also have pm_timer. But those beasts seem to have lost
>> that too.
>
> I suppose that one way of avoiding clock-skew messages is to have only
> one clock.

Indeed. It's a complete mystery why it takes ages to implement reliable
clocks in hardware.

>> >> We really need to remove the watchdog requirement for modern hardware.
>> >> Let me stare at those patches and get them merged.
>> >
>> > You are more trusting of modern hardware than I am, but for all I know,
>> > maybe rightfully so.  ;-)
>> 
>> Well, I rather put a bet on the hardware, which has become reasonable
>> over the last decade, than on trying to solve a circular dependency
>> problem with tons of heuristics which won't ever work correctly.
>
> Use of HPET to check the interval length would not be circular, right?

As long as the HPET works reliably :)

>> TSC_ADJUST is a reasonable safety net and since its invention the amount
>> of BIOS wreckage has been massively reduced. Seems the nastigram in
>> dmesg when detecting a change in TSC_ADJUST had an effect or maybe
>> Microsoft enforces a tinkerfree TSC by now and we get the benefit. :)
>> 
>> I still wish to have a knob to lock down TSC to read only, but that's
>> probably for christmas 2030 or later. :)
>
> Indeed.  How would BIOS writers hide their SMI handlers?  :-/

TSC_ADJUST already ruined that party.

Thanks,

        tglx