questions about header.S

questions about header.S

[Cao jin]

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

Re: questions about header.S

[Cao jin]

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.
> 

Re: questions about header.S

[Thomas Gleixner]

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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.

> > 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.

Thanks,

	tglx

Re: questions about header.S

[Cao jin]

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