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 15:37:26 -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 Wed, Dec 01, 2021 at 12:19:43AM +0100, Thomas Gleixner wrote:
> 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.

That one is easy.  It is because the previous clocksource watchdog was
too lenient.  ;-)

(Sorry, couldn't resist...)

> >> >> 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 :)

Is it also a complete mystery why clocksources previously deemed
reliable no longer work 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.

Give the BIOS writers time, they will figure something else out.  :-(

							Thanx, Paul