linux-kernel - Re: [patch 1/2] x86: head_64.S cleanup - use straight move to CR4 register

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Date:	Tue, 13 May 2008 10:11:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	jirislaby@...il.com
Subject: Re: [patch 1/2] x86: head_64.S cleanup - use straight move to CR4
 register

Cyrill Gorcunov wrote:
> [H. Peter Anvin - Tue, May 13, 2008 at 10:03:42AM -0700]
>> Cyrill Gorcunov wrote:
>>> There is no need for testing the values because we already know
>>> what they should be. Just set them in straight way.
>> He isn't testing them, he's setting individual bits.
>>
>> Either of these is pretty silly; the right way to do this is:
>>
>> 	movl	$(X86_CR4_PAE|X86_CR4_PGE), %eax
>> 	movq	%rax, %cr4
>>
>> A movl in 64-bit mode zero extends.
>>
>> 	-hpa
>>
> 
> Here is updated version
> ---
> [PATCH] x86: head_64.S cleanup - use straight move to CR4 register
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@...il.com>
> ---
> 
> Index: linux-2.6.git/arch/x86/kernel/head_64.S
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/kernel/head_64.S	2008-05-13 20:04:23.000000000 +0400
> +++ linux-2.6.git/arch/x86/kernel/head_64.S	2008-05-13 21:08:20.000000000 +0400
> @@ -155,9 +155,7 @@ ENTRY(secondary_startup_64)
>  	 */
>  
>  	/* Enable PAE mode and PGE */
> -	xorq	%rax, %rax
> -	btsq	$5, %rax
> -	btsq	$7, %rax
> +	movl	$(X86_CR4_PAE | X86_CR4_PGE), %rax
>  	movq	%rax, %cr4
>  

Syntax error.  %eax, not %rax.

	-hpa
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