Re: [PATCH v7 6/8] x86: tsc: Always Running Timer (ART) correlated clocksource

[Christopher Hall]

On Thu, 18 Feb 2016 13:11:33 -0800, Andy Lutomirski <luto@xxxxxxxxxx>  
wrote:
> > +#define X86_FEATURE_INVARIANT_TSC (7*32+4) /* Intel Invariant TSC */

This is removed. It was basically an alias for NONSTOP_TSC and not needed.

> > +/*
> > + * Convert ART to TSC given numerator/denominator found in detect_art()
> > + */
> > +struct system_counterval_t convert_art_to_tsc(cycle_t art)
> > +{
> > +	u64 tmp, res, rem;
> > +
> > +	rem = do_div(art, art_to_tsc_denominator);
> > +
> > +	res = art * art_to_tsc_numerator;
> > +	tmp = rem * art_to_tsc_numerator;
> > +
> > +	do_div(tmp, art_to_tsc_denominator);
> > +	res += tmp;
> > +
> > +	return (struct system_counterval_t) {.cs = art_related_clocksource,
> > +			.cycles = res};
>
> The SDM and the patch description both mention an offset "k".  Shouldn't  
> this code at least have a comment about how it deals with the k != 0  
> case?

I don't deal with the k != 0 case. I assume that IA32 TSC adjust MSR is 0  
because it's almost always a *bad idea* to change it. I've discussed this  
with a few other developers and there is some consensus agreeing. From an  
earlier related thread Peter Zijlstra asserts that TSC adjust "had
better" be 0.(http://lkml.iu.edu/hypermail/linux/kernel/1507.3/03734.html).

Do we really need to accommodate BIOS's that do this?

Chris