linux-kernel - Re: [PATCHv5 06/19] x86/boot/compressed/64: Detect and handle 5-level paging at boot-time

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Date:   Sun, 27 Aug 2017 14:29:26 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Borislav Petkov <bp@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 06/19] x86/boot/compressed/64: Detect and handle
 5-level paging at boot-time

On Mon, Aug 21, 2017 at 06:29:03PM +0300, Kirill A. Shutemov 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 fbf4c32d0b62..2e362aea3319 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -347,6 +347,28 @@ preferred_addr:
>  	leaq	boot_stack_end(%rbx), %rsp
>  
>  #ifdef CONFIG_X86_5LEVEL
> +	/* Preserve rbx across cpuid */
> +	movq	%rbx, %r8
> +
> +	/* Check if leaf 7 is supported */
> +	movl	$0, %eax

Use xor instead, it should be shorter

> +	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
> +	movl	$0, %ecx

same