Re: [RFC PATCH 5/8] x86/umwait.c: Add sysfs interface to show tsc_khz

[Fenghua Yu <fenghua.yu@intel.com>]

On Tue, Jun 19, 2018 at 11:08:46AM +0200, Thomas Gleixner wrote:
> On Fri, 15 Jun 2018, Fenghua Yu wrote:
>  
> > +static ssize_t tsc_khz_show(struct device *dev, struct device_attribute *attr,
> > +			    char *buf)
> > +{
> > +	return sprintf(buf, "%d\n", tsc_khz);
> > +}
> > +
> >  static DEVICE_ATTR_RW(umwait_disable_c0_2);
> > +static DEVICE_ATTR_RO(tsc_khz);
> 
> The right place to expose that information is the VDSO and a helper
> function which allows to convert from nsec to TSC.

Sure. I will do that.

> 
> >  static struct attribute *umwait_attrs[] = {
> >  	&dev_attr_umwait_disable_c0_2.attr,
> > @@ -92,6 +100,15 @@ static int __init umwait_init(void)
> >  	if (ret)
> >  		return ret;
> >  
> > +	/* Only add the tsc_khz interface when the value is known. */
> 
> Why so? The only reason why you don't want to expose TSC frequency is when
> it's not constant frequency.

> > +	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
> 

Thanks.

-Fenghua