linux-kernel - Re: [PATCHv7 06/19] x86/boot/compressed/64: Detect and handle 5-level paging at boot-time

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Date:   Thu, 28 Sep 2017 10:18:25 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 06/19] x86/boot/compressed/64: Detect and handle
 5-level paging at boot-time


* Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:

> This patch prepare decompression code to boot-time switching between 4-
> and 5-level paging.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/boot/compressed/head_64.S | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index b4a5d284391c..09c85e8558eb 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -288,6 +288,28 @@ ENTRY(startup_64)
>  	leaq	boot_stack_end(%rbx), %rsp
>  
>  #ifdef CONFIG_X86_5LEVEL
> +	/* Preserve rbx across cpuid */
> +	movq	%rbx, %r8
> +
> +	/* Check if leaf 7 is supported */
> +	xorl	%eax, %eax
> +	cpuid
> +	cmpl	$7, %eax
> +	jb	lvl5
> +
> +	/*
> +	 * Check if la57 is supported.
> +	 * The feature is enumerated with CPUID.(EAX=07H, ECX=0):ECX[bit 16]
> +	 */
> +	movl	$7, %eax
> +	xorl	%ecx, %ecx
> +	cpuid
> +	andl	$(1 << 16), %ecx
> +	jz	lvl5
> +
> +	/* Restore rbx */

In (new) x86 asm code we refer to registers in capital letters.

Also, CPUID should be capitalized consistently as well.

Also, LA57 should be capitalized as well.

> +	movq	%r8, %rbx
> +
>  	/* Check if 5-level paging has already enabled */
>  	movq	%cr4, %rax

BTW., please also fix the typo in this comment while at it.

Thanks,

	Ingo