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SubjectRe: [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@zytor.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@zytor.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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