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

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Date:   Thu, 13 Oct 2016 17:18:36 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     Grzegorz Andrejczuk <grzegorz.andrejczuk@...el.com>,
        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 Wed, 12 Oct 2016, Dave Hansen wrote:

> 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.

Indeed. It came to me after sending the mail and closing the notebook to
head out for more conference fun. I expected someone to notice it :)
 
> 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.

Right, and we should do the following:

		__u8		x86;
		__u8		x86_vendor;
		__u8		x86_model;
		__u8		x86_mask;
		u32		x86_fvm;

set x86_fvm to family | vendor << 8 | model << 16; and then do the
comparison on that instead of checking 3 bytes in a row.

Thanks,

	tglx