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

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Date:	Wed, 18 Jun 2014 19:21:11 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Qiaowei Ren <qiaowei.ren@...el.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 09:25:57AM -0700, Dave Hansen wrote:
> On 06/18/2014 07:59 AM, H. Peter Anvin wrote:
> > On 06/18/2014 07:35 AM, Dave Hansen wrote:
> >> 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().
> >>
> >> So, we can either use the well worn, consistent with other features in
> >> x86, cpu_has_$foo approach.  Or, we can roll our own macros.
> > 
> > We could do something like:
> > 
> > #define MPX_ENABLED (IS_ENABLED(CONFIG_X86_MPX) &&
> > static_cpu_has(X86_FEATURE_MPX))
> 
> How about something like the attached patch?
> 
> This lets us use static_cpu_has() for the checks, and allows us to
> easily add new checks for other features that might be compile-time
> disabled.

> 
> 
> ---
> 
>  b/arch/x86/include/asm/cpufeature.h |   26 ++++++++++++++++++++------
>  b/arch/x86/kernel/mpx.c             |    4 ++--
>  b/arch/x86/kernel/traps.c           |    2 +-
>  3 files changed, 23 insertions(+), 9 deletions(-)
> 
> diff -puN arch/x86/include/asm/cpufeature.h~x86-disabled_mask arch/x86/include/asm/cpufeature.h
> --- a/arch/x86/include/asm/cpufeature.h~x86-disabled_mask	2014-06-18 08:48:41.329750895 -0700
> +++ b/arch/x86/include/asm/cpufeature.h	2014-06-18 09:19:19.143546973 -0700
> @@ -339,12 +339,6 @@ extern const char * const x86_power_flag
>  #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)
> -#else
> -#define cpu_has_mpx 0
> -#endif /* CONFIG_X86_INTEL_MPX */
> -
>  #ifdef CONFIG_X86_64
>  
>  #undef  cpu_has_vme
> @@ -367,6 +361,22 @@ extern const char * const x86_power_flag
>  
>  #endif /* CONFIG_X86_64 */
>  
> +/*
> + * Add features and their corresponding config options here
> + * if you want to have the compiler optimize out code that
> + * uses them.
> + *
> + * You should not use this function directly.  Use
> + * static_cpu_has() so that you also benefit from alternatives
> + * when the features are enabled.
> + */
> +static __always_inline int __cpu_feature_compile_enabled(u16 bit)
> +{
> +	if (bit == X86_FEATURE_MPX)
> +		return IS_ENABLED(CONFIG_X86_MPX);

Right, this should be CONFIG_X86_INTEL_MPX but that's details.

I guess this might grow into a big if-else noodle case but when that
happens, we probably could add some sort of mapping between X86_FEATURE
bits to CONFIG_ items or so.

Then, you could probably add this call to the enclosing macros
static_cpu_has{,_safe} so that it gets evaluated first. The advantage is
that the IS_ENABLED test would work very early - always, actually - even
before alternatives have run and thus catch the cases where the feature
is config-disabled.

Otherwise, asm looks ok.

-- 
Regards/Gruss,
    Boris.

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