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

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Message-ID: <20180619151131.GA112652@romley-ivt3.sc.intel.com>
Date:   Tue, 19 Jun 2018 08:11:32 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, 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 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