linux-kernel - questions about header.S

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Date:   Sat, 17 Mar 2018 18:01:27 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: questions about header.S

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.

-- 
Sincerely,
Cao jin