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

[Thomas Gleixner]

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.

>  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,

	tglx