Re: [PATCH 2/33] i386: define __pa_symbol

From: Andi Kleen
Date: Tue Aug 01 2006 - 15:04:14 EST

"Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> writes:

> On x86_64 we have to be careful with calculating the physical
> address of kernel symbols. Both because of compiler odditities
> and because the symbols live in a different range of the virtual
> address space.
> Having a defintition of __pa_symbol that works on both x86_64 and
> i386 simplifies writing code that works for both x86_64 and
> i386 that has these kinds of dependencies.
> So this patch adds the trivial i386 __pa_symbol definition.
> Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> ---
> include/asm-i386/page.h | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
> diff --git a/include/asm-i386/page.h b/include/asm-i386/page.h
> index f5bf544..eceb7f5 100644
> --- a/include/asm-i386/page.h
> +++ b/include/asm-i386/page.h
> @@ -124,6 +124,7 @@ #define PAGE_OFFSET ((unsigned long)__P
> #define VMALLOC_RESERVE ((unsigned long)__VMALLOC_RESERVE)
> #define __pa(x) ((unsigned long)(x)-PAGE_OFFSET)
> +#define __pa_symbol(x) __pa(x)

Actually PAGE_OFFSET arithmetic on symbols is outside ISO C and gcc
misoptimizes it occassionally. You would need to use HIDE_RELOC
or similar. That is why x86-64 has the magic.

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