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

[Thomas Gleixner]

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