linux-kernel - Re: [PATCH] ARM: Sort exception table at compile time

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Date:	Mon, 30 Jul 2012 23:51:18 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH] ARM: Sort exception table at compile time

On Mon, Jul 30, 2012 at 02:30:03PM -0700, Stephen Boyd wrote:
> Add the ARM machine identifier to sortextable and select the
> config option so that we can sort the exception table at compile
> time. sortextable relies on a section named __ex_table existing
> in the vmlinux, but ARM's linker script places the exception
> table in the data section. Give the exception table its own
> section so that sortextable can find it.
> 
> This allows us to skip the runtime sorting step during boot.
> 
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> Cc: David Daney <david.daney@...ium.com>
> ---
> 
> I can't find any information on why the exception table lives in the data
> section. If there's a good reason for that, I'll look into changing
> sortextable to look for the __start___ex_table symbol.
> 
>  arch/arm/Kconfig              |  1 +
>  arch/arm/kernel/vmlinux.lds.S | 19 +++++++++----------
>  scripts/sortextable.c         |  1 +
>  3 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index b25c9d3..2af95e6 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1,6 +1,7 @@
>  config ARM
>  	bool
>  	default y
> +	select BUILDTIME_EXTABLE_SORT if MMU
>  	select ARCH_HAVE_CUSTOM_GPIO_H
>  	select HAVE_AOUT
>  	select HAVE_DMA_API_DEBUG
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index 36ff15b..0e3e8b4 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -220,16 +220,6 @@ SECTIONS
>  		READ_MOSTLY_DATA(L1_CACHE_BYTES)
>  
>  		/*
> -		 * The exception fixup table (might need resorting at runtime)
> -		 */
> -		. = ALIGN(4);
> -		__start___ex_table = .;
> -#ifdef CONFIG_MMU
> -		*(__ex_table)
> -#endif
> -		__stop___ex_table = .;
> -
> -		/*
>  		 * and the usual data section
>  		 */
>  		DATA_DATA
> @@ -239,6 +229,15 @@ SECTIONS
>  	}
>  	_edata_loc = __data_loc + SIZEOF(.data);
>  
> +	. = ALIGN(4);
> +	__ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
> +		__start___ex_table = .;
> +#ifdef CONFIG_MMU
> +		*(__ex_table)
> +#endif
> +		__stop___ex_table = .;
> +	}
> +

Here you should be able to use the macro EXCEPTION_TABLE from
vmlinux.lds.h.
I cannot see why the ifdef for not NON-MMU case is needed,
but if it is needed the macro is not good...

	Sam
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