linux-kernel - Re: [PATCH v6 03/10] x86, mpx: add macro cpu_has

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Message-ID: <20140618144429.GD24024@pd.tnic>
Date:	Wed, 18 Jun 2014 16:44:29 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Qiaowei Ren <qiaowei.ren@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 03/10] x86, mpx: add macro cpu_has_mpx

On Wed, Jun 18, 2014 at 07:35:17AM -0700, Dave Hansen wrote:
> On 06/18/2014 02:57 AM, Borislav Petkov wrote:
> >> > @@ -339,6 +339,12 @@ extern const char * const x86_power_flags[32];
> >> >  #define cpu_has_eager_fpu	boot_cpu_has(X86_FEATURE_EAGER_FPU)
> >> >  #define cpu_has_topoext		boot_cpu_has(X86_FEATURE_TOPOEXT)
> >> >  
> >> > +#ifdef CONFIG_X86_INTEL_MPX
> >> > +#define cpu_has_mpx boot_cpu_has(X86_FEATURE_MPX)
> > I think we don't want those macros anymore because they're obfuscating
> > the code. You should use static_cpu_has instead.
> 
> It looks like static_cpu_has() is the right thing to use instead of
> boot_cpu_has().  But, this doesn't just obfuscate things.
> 
> We actually _want_ the compiler to cull code out when the config option
> is off.  Things like do_bounds() will see code savings with _some_ kind
> of #ifdef rather than using static_cpu_has().

Why?

Practically, distros will have it enabled anyway (you have X86_INTEL_MPX
depend on CPU_SUP_INTEL).

Are you talking about the miniscule percentage of people building their
own kernels?

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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