linux-kernel - Re: [PATCH] x86: avoid warning for zero-filling .bss

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Date:   Thu, 17 Nov 2016 13:19:22 -0800
From:   hpa@...or.com
To:     Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>
CC:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: avoid warning for zero-filling .bss

On November 17, 2016 1:02:48 PM PST, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>On Wed, Nov 16, 2016 at 03:17:09PM +0100, Arnd Bergmann wrote:
>> The latest binutils are warning about a .fill directive with an
>explicit
>> value in a .bss section:
>> 
>> arch/x86/kernel/head_32.S: Assembler messages:
>> arch/x86/kernel/head_32.S:677: Warning: ignoring fill value in
>section `.bss..page_aligned'
>> arch/x86/kernel/head_32.S:679: Warning: ignoring fill value in
>section `.bss..page_aligned'
>> 
>> This comes from the 'ENTRY()' macro padding the space between the
>symbols
>> with 'nop'. Open-coding the .globl directive without the padding
>> avoids that warning.
>
>How is there space between the symbols?  Aren't they already aligned?
>
>Isn't the warning really about the fact that it's unnecessarily filling
>a .bss section with zeros?  Would it make sense to use .align instead?
>
>> ---
>>  arch/x86/kernel/head_32.S | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
>> index df541ac2071e..4e8577d03372 100644
>> --- a/arch/x86/kernel/head_32.S
>> +++ b/arch/x86/kernel/head_32.S
>> @@ -669,14 +669,17 @@ __PAGE_ALIGNED_BSS
>>  initial_pg_pmd:
>>  	.fill 1024*KPMDS,4,0
>>  #else
>> -ENTRY(initial_page_table)
>> +.globl initial_page_table
>> +initial_page_table:
>>  	.fill 1024,4,0
>>  #endif
>>  initial_pg_fixmap:
>>  	.fill 1024,4,0
>> -ENTRY(empty_zero_page)
>> +.globl empty_zero_page
>> +empty_zero_page:
>>  	.fill 4096,1,0
>> -ENTRY(swapper_pg_dir)
>> +.globl swapper_pg_dir
>> +swapper_pg_dir:
>>  	.fill 1024,4,0
>>  EXPORT_SYMBOL(empty_zero_page)
>>  
>> -- 
>> 2.9.0
>> 

Use .org, .align, or .space
-- 
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