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

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Date:   Tue, 30 Nov 2021 22:55:45 +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 12:47, Paul E. McKenney wrote:
> On Tue, Nov 30, 2021 at 09:39:32PM +0100, Thomas Gleixner wrote:
>> Seriously. Jiffies is not usable as watchdog simply because lost ticks
>> cannot be compensated and you cannot use TSC to bridge them because you
>> are not trusting TSC. This is simply a circulus vitiosus.
>
> OK, HPET or nothing, then.

Older machines also have pm_timer. But those beasts seem to have lost
that too.

>> 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.

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. :)

Thanks,

        tglx