Re: [PATCH] x86/tsc: Add tsc_tuned_baseclk flag disabling CPUID.16h use for tsc calibration

[Krzysztof Piecuch]

On Monday, January 20, 2020 1:42 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Simply because all of this is horribly fragile and if you put virt into
> the picture it gets even worse.
>
> The initial calibration via PIT/HPET is halfways accurate in most cases
> and we use the 1% as a sanity check.
>
> > Ideally it would be better to get the early calibration right than
> > risk getting it wrong because of an "anomaly".
>
> Ideally we would just have a way to read the stupid frequency from some
> reliable place, but there is no such thing.
>
> Guess why we have all this code, surely not because we have nothing
> better to do than dreaming up a variety of weird ways to figure out that
> frequency.

Thank you for the explanation.

> Widening the error window here is clearly a hack. As you have to supply
> a valid number there, then why not just providing the frequency itself
> on the command line? That would at least make most sense and would avoid
> to use completely wrong data in the early boot stage.

That sounds good.
I'll assume that the user will be supposed to provide a flag tsc_early_hz=
so that refine_calibration_work can get better numbers while still doing
the 1% sanity check.

I'll send a patch this week.