linux-kernel - Re: questions about header.S

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Date:   Wed, 21 Mar 2018 17:23:28 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: questions about header.S

Dear Maintainers,

  Could you help to give a hint?
  Thanks in advance.

-- 
Sincerely,
Cao jin

On 03/17/2018 06:01 PM, Cao jin wrote:
> Hi,
> 
>   I find two small questions which confuse me a little.
> 
> 1.
> # Check signature at end of setup
>        cmpl    $0x5a5aaa55, setup_sig
>        jne     setup_bad
> 
> setup_sig is defined in setup.ld, which points to the constant also
> defined in setup.ld, so I don't figure out in which case they don't
> equal and jump to setup_bad?
> 
> In my test, drop these 2 lines seems fine, system can boot without any
> obvious error.
> 
> 2.
> # Zero the bss
> 	movw	$__bss_start, %di
> 	movw	$_end+3, %cx
> 	xorl	%eax, %eax
> 	subw	%di, %cx
> 	shrw	$2, %cx
> 	rep; stosl
> 
> It is not a big deal, but I think replace "_end" with "__bss_end" make
> more sense, and "_end" is already aligned to word length. And, there is
> no other code use symbol "__bss_end". So I don't know is there any
> reason to use "_end" here?
> 
> In my test, replace the 2nd line with:
> 
> 	movw	$_end, %cx
> 
> or:
> 
> 	movw	$__bss_end+3, %cx
> 
> are both fine.
>