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

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Date:   Thu, 17 Nov 2016 15:02:48 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: avoid warning for zero-filling .bss

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
> 

-- 
Josh