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 21:09:56 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"H. Peter Anvin" <hpa@...or.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

[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
 
 	/* Setup early boot stage 4 level pagetables. */
--
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