linux-kernel - Re: [PATCH 2/2] x86/head_64.S: Pass struct boot

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Date:   Tue, 7 Mar 2017 10:11:40 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] x86/head_64.S: Pass struct boot_params' virtual
 address to C


* Borislav Petkov <bp@...en8.de> wrote:

> From: Borislav Petkov <bp@...e.de>
> 
> ... so that callees don't have to convert it and can use it directly.
> Simplifies C code a bit. Cleanup comments and formatting in the
> vicinity, while at it.
> 
> Also, document what phys_base really is.
> 
> No functionality change.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>  arch/x86/kernel/head64.c  |  8 ++++----
>  arch/x86/kernel/head_64.S | 10 +++++++---
>  2 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 54a2372f5dbb..17ee20a9b4e4 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -118,7 +118,7 @@ static unsigned long get_cmd_line_ptr(void)
>  
>  static void __init copy_bootdata(char *real_mode_data)
>  {
> -	char * command_line;
> +	char *command_line;
>  	unsigned long cmd_line_ptr;
>  
>  	memcpy(&boot_params, real_mode_data, sizeof boot_params);
> @@ -130,7 +130,7 @@ static void __init copy_bootdata(char *real_mode_data)
>  	}
>  }
>  
> -asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
> +asmlinkage __visible void __init x86_64_start_kernel(char *real_mode_data)
>  {
>  	int i;
>  
> @@ -163,7 +163,7 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
>  		set_intr_gate(i, early_idt_handler_array[i]);
>  	load_idt((const struct desc_ptr *)&idt_descr);
>  
> -	copy_bootdata(__va(real_mode_data));
> +	copy_bootdata(real_mode_data);
>  
>  	/*
>  	 * Load microcode early on BSP.
> @@ -180,7 +180,7 @@ void __init x86_64_start_reservations(char *real_mode_data)
>  {
>  	/* version is always not zero if it is copied */
>  	if (!boot_params.hdr.version)
> -		copy_bootdata(__va(real_mode_data));
> +		copy_bootdata(real_mode_data);
>  
>  	x86_early_init_platform_quirks();
>  
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index ac9d327d2e42..506de36293bc 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -266,9 +266,12 @@ ENTRY(secondary_startup_64)
>  	movl	initial_gs+4(%rip),%edx
>  	wrmsr
>  
> -	/* rsi is pointer to real mode structure with interesting info.
> -	   pass it to C */
> -	movq	%rsi, %rdi
> +	/*
> +	 * %rsi is pointer to real mode struct boot_params with interesting info.
> +	 * Pass its virtual address to C.
> +	 */
> +	movq	$__PAGE_OFFSET_BASE, %rdi
> +	addq	%rsi, %rdi

The updated comments and other details are fine, but I'm not sure about the __va() 
change: the patch essentially open codes __va() in assembly - which would make any 
changes to __va() [for whatever reason] more difficult, right?

Right now we don't seem to have a single such line of assembly, so changes to 
__va() can be done in C alone.

Thanks,

	Ingo