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 14:48:25 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 10:55:45PM +0100, Thomas Gleixner wrote:
> 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.

I suppose that one way of avoiding clock-skew messages is to have only
one clock.

> >> 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?

> 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?  :-/

							Thanx, Paul