linux-kernel - Re: [PATCH 4/4] x86/cpu: Provide a config option to disable static_cpu

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Message-ID: <20151201162444.GC26089@x>
Date:	Tue, 1 Dec 2015 08:24:45 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] x86/cpu: Provide a config option to disable
 static_cpu_has

On Tue, Dec 01, 2015 at 04:00:23PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> This brings .text savings of about ~1.6K when building a tinyconfig. It
> is off by default so nothing changes for the default.
> 
> Kconfig help text from Josh.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> Cc: Josh Triplett <josh@...htriplett.org>

Assuming this passes the kernel build robot, then:
Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  arch/x86/Kconfig                  | 11 +++++++++++
>  arch/x86/include/asm/cpufeature.h |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index db3622f22b61..a2abc2fb6970 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -349,6 +349,17 @@ config X86_FEATURE_NAMES
>  
>  	  If in doubt, say Y.
>  
> +config X86_FAST_FEATURE_TESTS
> +	bool "Fast CPU feature tests" if EMBEDDED
> +	default y
> +	---help---
> +	  Some fast-paths in the kernel depend on the capabilities of the CPU.
> +	  Say Y here for the kernel to patch in the appropriate code at runtime
> +	  based on the capabilities of the CPU. The infrastructure for patching
> +	  code at runtime takes up some additional space; space-constrained
> +	  embedded systems may wish to say N here to produce smaller, slightly
> +	  slower code.
> +
>  config X86_X2APIC
>  	bool "Support x2apic"
>  	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index 144b042c0872..43e144474043 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -409,7 +409,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
>   * fast paths and boot_cpu_has() otherwise!
>   */
>  
> -#if __GNUC__ >= 4
> +#if __GNUC__ >= 4 && defined(CONFIG_X86_FAST_FEATURE_TESTS)
>  extern void warn_pre_alternatives(void);
>  extern bool __static_cpu_has_safe(u16 bit);
>  
> -- 
> 2.3.5
> 
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