linux-kernel - Re: [PATCH 8/8] x86, cleanups: Simplify sync

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Message-ID: <20121128204158.GC14635@liondog.tnic>
Date:	Wed, 28 Nov 2012 21:41:58 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mario Gzuk <mariogzuk@...hnikz.de>
Subject: Re: [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case of
 no CPUID

On Wed, Nov 28, 2012 at 11:50:30AM -0800, H. Peter Anvin wrote:
> From: "H. Peter Anvin" <hpa@...ux.intel.com>
> 
> Simplify the implementation of sync_core() for the case where we may
> not have the CPUID instruction available.
> 
> Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
> ---
>  arch/x86/include/asm/processor.h | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 9a4ee46..b381df7 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -673,17 +673,24 @@ static inline void sync_core(void)
>  	int tmp;
>  
>  #ifdef CONFIG_M486
> -	if (boot_cpu_data.x86 < 5)
> -		/* There is no speculative execution.
> -		 * jmp is a barrier to prefetching. */
> -		asm volatile("jmp 1f\n1:\n" ::: "memory");
> -	else
> +	/*
> +	 * Do a CPUID if available, otherwise do a jump.  The jump
> +	 * can conveniently enough be the jump around CPUID.
> +	 */
> +	asm volatile("cmpl %2,%1\n\t"
> +		     "jl 1f\n\t"
> +		     "cpuid\n"
> +		     "1:"
> +		     : "=a" (tmp)
> +		     : "rm" (boot_cpu_data.cpuid_level), "ri" (0), "0" (1)
> +		     : "ebx", "ecx", "edx", "memory");
> +#else
> +	/* cpuid is a barrier to speculative execution.
> +	 * Prefetched instructions are automatically
> +	 * invalidated when modified. */

While at it, you could correct this comment to adhere to kernel coding
style:

	/*
	 * cpuid is a barrier...
	 * ...
	 */

> +	asm volatile("cpuid" : "=a" (tmp) : "0" (1)
> +		     : "ebx", "ecx", "edx", "memory");

... and then write this in its shorter form:

	tmp = cpuid_eax(1);

to have it a bit easier on the eyes.

Thanks.

-- 
Regards/Gruss,
    Boris.
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