Date:   Tue, 30 Nov 2021 15:37:26 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Feng Tang <feng.tang@intel.com>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        andi.kleen@intel.com, len.brown@intel.com, tim.c.chen@intel.com
Subject: Re: [PATCH v3 2/2] x86/tsc: skip tsc watchdog checking for qualified
 platforms
Message-ID: <20211130233726.GD641268@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20211117023751.24190-2-feng.tang@intel.com>
 <20211130064623.GB96474@shbuild999.sh.intel.com>
 <20211130144048.GQ641268@paulmck-ThinkPad-P17-Gen-1>
 <20211130150256.GA19477@shbuild999.sh.intel.com>
 <20211130162815.GU641268@paulmck-ThinkPad-P17-Gen-1>
 <87r1axbcor.ffs@tglx>
 <20211130204721.GZ641268@paulmck-ThinkPad-P17-Gen-1>
 <87ilw9b95q.ffs@tglx>
 <20211130224825.GA641268@paulmck-ThinkPad-P17-Gen-1>
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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