linux-kernel - Re: questions about header.S

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Date:   Thu, 22 Mar 2018 10:31:41 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: questions about header.S

Thanks very much for you hint!

On 03/21/2018 05:57 PM, Thomas Gleixner wrote:
> On Wed, 21 Mar 2018, Cao jin wrote:
>> On 03/17/2018 06:01 PM, Cao jin wrote:
>>>   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?
> 
> That's a lame sanity check to make sure that nothing overwrote the loader.
> 

I see.

>>> In my test, drop these 2 lines seems fine, system can boot without any
>>> obvious error.
> 
> Sure it does as long as you have no corruption.
> 
>>> 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?
> 
> It doesn't matter at all. But its also pointless to change it.
> 

It does not matter and is pointless to change for kernel itself. It just
may confuse a little for newbies who has interests.

-- 
Sincerely,
Cao jin