linux-kernel - Re: [PATCH v2 2/4] Add enabling of the R3 MWAIT during boot for KNL

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Date:   Wed, 12 Oct 2016 09:28:58 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>
Cc:     mingo@...hat.com, hpa@...or.com, x86@...nel.org, bp@...e.de,
        linux-kernel@...r.kernel.org, lukasz.daniluk@...el.com,
        james.h.cownie@...el.com
Subject: Re: [PATCH v2 2/4] Add enabling of the R3 MWAIT during boot for KNL

On 10/12/2016 06:34 AM, Thomas Gleixner wrote:
>> > +	if (c->x86 == 6 &&
>> > +	    c->x86_model == INTEL_FAM6_XEON_PHI_KNL &&
>> > +	    phir3mwait) {
>> > +		u64 prev;
>> > +
>> > +		rdmsrl(MSR_PHI_MISC_THD_FEATURE, prev);
>> > +		if ((prev & MSR_PHI_MISC_THD_FEATURE_R3MWAIT) == 0)
>> > +			wrmsrl(MSR_PHI_MISC_THD_FEATURE,
>> > +			       prev | MSR_PHI_MISC_THD_FEATURE_R3MWAIT);
> The codingstyle here is just convoluted crap. What's wrong with writing it
> proper?
> 
> 	if (c->x86_model == INTEL_FAM6_XEON_PHI_KNL && phir3mwait) {
> 		u64 msr;
> 
> 		rdmsrl(MSR_PHI_MISC_THD_FEATURE, msr);
> 		msr |= MSR_PHI_MISC_THD_FEATURE_R3MWAIT;
> 		wrmsrl(MSR_PHI_MISC_THD_FEATURE, msr);
> 
> 	}
> 
> No horrible to read line breaks, no redundant check for x->x86 == 6 because
> model cannot be INTEL_FAM6_XEON_PHI_KNL if x->x86 != 6. Also the
> conditional is pointless as the feature is default disabled. And even if it
> is enabled the extra msr write is not a problem at all. This is early init
> code and not some hot path.

Hi Thomas,

We really do need to check for family=6 (c->x86==6).
INTEL_FAM6_XEON_PHI_KNL is just for the model and doesn't check family.
 It implies that you've already checked for family 6.

Looking at the name, though, it's pretty clear that the naming can
easily trip folks up.

I do think we've probably screwed up the way we use our 'struct
x86_cpu_id' mechanism.  Maybe we should be providing the
vendor/family/model sets from a common place to the drivers, instead of
making them all repeat it individually.

Like have a big header full of:

	DECLARE_CPU(INTEL_XEON_PHI_KNL, INTEL..., 6, MODEL_XYZ...);

Once we have that, everybody can just do:

	if(cpu_is(c, INTEL_XEON_PHI_KNL))
		...

and get all the checking they need.