linux-kernel - Re: [PATCH] Move -fno-dwarf2-cfi-asm to powerpc only

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Date:	Mon, 21 Dec 2009 15:39:10 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, kyle@...hat.com, mmarek@...e.cz
Subject: Re: [PATCH] Move -fno-dwarf2-cfi-asm to powerpc only

On Mon, 2009-12-21 at 02:23 +0100, Andi Kleen wrote:
> Move -fno-dwarf2-cfi-asm to powerpc only
> 
> Better dwarf2 unwind information is a good thing, it allows better
> debugging with kgdb and crash and helps systemtap.
> 
> commit 003086497f07f7f1e67c0c295e261740f822b377 disabled some CFI
> information globally to work around a module loader bug on powerpc.
> 
> But this disables the better unwind tables for all architectures,
> not just powerpc. Move the workaround to powerpc and also 
> add a suitable comment that's it really a workaround.
> 
> This improves dwarf2 unwind tables on x86 at least.

Heh, I don't remember the original problem but yeah, that's definitely
not something to keep in generic Makefiles and in fact we should fix the
problem on the powerpc side. I'll have a look.

Cheers,
Ben.

> Cc: kyle@...hat.com
> Cc: benh@...nel.crashing.org
> Cc: mmarek@...e.cz
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> 
> ---
>  Makefile              |    3 ---
>  arch/powerpc/Makefile |    5 +++++
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.33-rc1-ak/Makefile
> ===================================================================
> --- linux-2.6.33-rc1-ak.orig/Makefile
> +++ linux-2.6.33-rc1-ak/Makefile
> @@ -570,9 +570,6 @@ KBUILD_CFLAGS += $(call cc-option,-Wno-p
>  # disable invalid "can't wrap" optimizations for signed / pointers
>  KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
>  
> -# revert to pre-gcc-4.4 behaviour of .eh_frame
> -KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
> -
>  # conserve stack if available
>  KBUILD_CFLAGS   += $(call cc-option,-fconserve-stack)
>  
> Index: linux-2.6.33-rc1-ak/arch/powerpc/Makefile
> ===================================================================
> --- linux-2.6.33-rc1-ak.orig/arch/powerpc/Makefile
> +++ linux-2.6.33-rc1-ak/arch/powerpc/Makefile
> @@ -112,6 +112,11 @@ KBUILD_CFLAGS += $(call cc-option,-mspe=
>  # kernel considerably.
>  KBUILD_CFLAGS += $(call cc-option,-funit-at-a-time)
>  
> +# FIXME: the module load should be taught about the additional relocs
> +# generated by this.
> +# revert to pre-gcc-4.4 behaviour of .eh_frame
> +KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
> +
>  # Never use string load/store instructions as they are
>  # often slow when they are implemented at all
>  KBUILD_CFLAGS		+= -mno-string


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