[PATCH] module: ppc64 module CRC relocation fix causes perf issues

[Benjamin Herrenschmidt]

On Mon, 2013-07-15 at 14:04 +1000, Anton Blanchard wrote:
> Module CRCs are implemented as absolute symbols that get resolved by
> a linker script. We build an intermediate .o that contains an
> unresolved symbol for each CRC. genksysms parses this .o, calculates
> the CRCs and writes a linker script that "resolves" the symbols to
> the calc

Scott, can somebody from FSL test that on 32-bit and Ack it ?

Thanks !

Cheers,
Ben.

> Unfortunately the ppc64 relocatable kernel sees these CRCs as symbols
> that need relocating and relocates them at boot. Commit d4703aef
> (module: handle ppc64 relocating kcrctabs when CONFIG_RELOCATABLE=y)
> added a hook to reverse the bogus relocations. Part of this patch
> created a symbol at 0x0:
> 
> # head -2 /proc/kallsyms 
> 0000000000000000 T reloc_start
> c000000000000000 T .__start
> 
> This reloc_start symbol is causing lots of confusion to perf. It
> thinks reloc_start is a massive function that stretches from 0x0 to
> 0xc000000000000000 and we get various cryptic errors out of perf,
> including:
> 
> problem incrementing symbol count, skipping event
> 
> This patch removes the  reloc_start linker script label and instead
> defines it as PHYSICAL_START. We also need to wrap it with
> CONFIG_PPC64 because the ppc32 kernel can set a non zero
> PHYSICAL_START at compile time and we wouldn't want to subtract
> it from the CRCs in that case.
> 
> Signed-off-by: Anton Blanchard <anton at samba.org>
> Cc: <stable at kernel.org>
> ---
> 
> This bug was originally reported on Fedora 19 (3.9.x), so I've marked
> it for stable.
> 
> Index: b/arch/powerpc/include/asm/module.h
> ===================================================================
> --- a/arch/powerpc/include/asm/module.h
> +++ b/arch/powerpc/include/asm/module.h
> @@ -82,10 +82,9 @@ struct exception_table_entry;
>  void sort_ex_table(struct exception_table_entry *start,
>  		   struct exception_table_entry *finish);
>  
> -#ifdef CONFIG_MODVERSIONS
> +#if defined(CONFIG_MODVERSIONS) && defined(CONFIG_PPC64)
>  #define ARCH_RELOCATES_KCRCTAB
> -
> -extern const unsigned long reloc_start[];
> +#define reloc_start PHYSICAL_START
>  #endif
>  #endif /* __KERNEL__ */
>  #endif	/* _ASM_POWERPC_MODULE_H */
> Index: b/arch/powerpc/kernel/vmlinux.lds.S
> ===================================================================
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -38,9 +38,6 @@ jiffies = jiffies_64 + 4;
>  #endif
>  SECTIONS
>  {
> -	. = 0;
> -	reloc_start = .;
> -
>  	. = KERNELBASE;
>  
>  /*