linux-kernel - Re: [PATCH 01/14] x86, boot: align the .bss section in the decompressor

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Date:	Fri, 8 May 2009 09:17:59 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, vgoyal@...hat.com, hbabu@...ibm.com,
	kexec@...ts.infradead.org, ying.huang@...el.com, mingo@...e.hu,
	tglx@...utronix.de, ebiederm@...ssion.com,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 01/14] x86, boot: align the .bss section in the decompressor

On Thu, May 07, 2009 at 03:26:49PM -0700, H. Peter Anvin wrote:
> From: H. Peter Anvin <hpa@...or.com>
> 
> Aligning the .bss section makes it trivially faster, and makes using
> larger transfers for the clear slightly easier.
> 
> [ Impact: trivial performance enhancement, future patch prep ]
> 
> Signed-off-by: H. Peter Anvin <hpa@...or.com>
> ---
>  arch/x86/boot/compressed/vmlinux.lds.S |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 0d26c92..27c168d 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -42,6 +42,7 @@ SECTIONS
>  		*(.data.*)
>  		_edata = . ;
>  	}
> +	. = ALIGN(32);

Where does this magic 32 comes from?
I would assume the better choice would be:
	. = ALIGN(L1_CACHE_BYTES);

So we match the relevant CPU.

In general for alignmnet of output sections I see the need for:
1) Function call
2) L1_CACHE_BYTES
3) PAGE_SIZE
4) 2*PAGE_SIZE

But I see magic constant used here and there that does not match
the above (when looking at all archs).
So I act when I see a new 'magic' number..

	Sam
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