linux-kernel - Re: [RFC PATCH 5/8] x86/umwait.c: Add sysfs interface to show tsc

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Date:   Tue, 19 Jun 2018 11:08:46 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 5/8] x86/umwait.c: Add sysfs interface to show
 tsc_khz

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