linux-kernel - Re: [PATCH 3/8] x86/boot/64: Add support of additional page table level during early boot

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Date:   Tue, 11 Apr 2017 09:02:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] x86/boot/64: Add support of additional page table
 level during early boot


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

> This patch adds support for 5-level paging during early boot.
> It generalizes boot for 4- and 5-level paging on 64-bit systems with
> compile-time switch between them.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/boot/compressed/head_64.S          | 23 ++++++++++++---
>  arch/x86/include/asm/pgtable_64.h           |  2 ++
>  arch/x86/include/uapi/asm/processor-flags.h |  2 ++
>  arch/x86/kernel/head64.c                    | 44 +++++++++++++++++++++++++----
>  arch/x86/kernel/head_64.S                   | 29 +++++++++++++++----
>  5 files changed, 85 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index d2ae1f821e0c..3ed26769810b 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -122,9 +122,12 @@ ENTRY(startup_32)
>  	addl	%ebp, gdt+2(%ebp)
>  	lgdt	gdt(%ebp)
>  
> -	/* Enable PAE mode */
> +	/* Enable PAE and LA57 mode */
>  	movl	%cr4, %eax
>  	orl	$X86_CR4_PAE, %eax
> +#ifdef CONFIG_X86_5LEVEL
> +	orl	$X86_CR4_LA57, %eax
> +#endif
>  	movl	%eax, %cr4
>  
>   /*
> @@ -136,13 +139,24 @@ ENTRY(startup_32)
>  	movl	$(BOOT_INIT_PGT_SIZE/4), %ecx
>  	rep	stosl
>  
> +	xorl	%edx, %edx
> +
> +	/* Build Top Level */
> +	leal	pgtable(%ebx,%edx,1), %edi
> +	leal	0x1007 (%edi), %eax
> +	movl	%eax, 0(%edi)
> +
> +#ifdef CONFIG_X86_5LEVEL
>  	/* Build Level 4 */
> -	leal	pgtable + 0(%ebx), %edi
> +	addl	$0x1000, %edx
> +	leal	pgtable(%ebx,%edx), %edi
>  	leal	0x1007 (%edi), %eax
>  	movl	%eax, 0(%edi)
> +#endif
>  
>  	/* Build Level 3 */
> -	leal	pgtable + 0x1000(%ebx), %edi
> +	addl	$0x1000, %edx
> +	leal	pgtable(%ebx,%edx), %edi
>  	leal	0x1007(%edi), %eax
>  	movl	$4, %ecx
>  1:	movl	%eax, 0x00(%edi)
> @@ -152,7 +166,8 @@ ENTRY(startup_32)
>  	jnz	1b
>  
>  	/* Build Level 2 */
> -	leal	pgtable + 0x2000(%ebx), %edi
> +	addl	$0x1000, %edx
> +	leal	pgtable(%ebx,%edx), %edi
>  	movl	$0x00000183, %eax
>  	movl	$2048, %ecx
>  1:	movl	%eax, 0(%edi)

I realize that you had difficulties converting this to C, but it's not going to 
get any easier in the future either, with one more paging mode/level added!

If you are stuck on where it breaks I'd suggest doing it gradually: first add a 
trivial .c, build and link it in and call it separately. Then once that works, 
move functionality from asm to C step by step and test it at every step.

I've applied the first two patches of this series, but we really should convert 
this assembly bit to C too.

Thanks,

	Ingo