linux-kernel - Re: [PATCH 2/33] i386: define __pa

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Message-ID: <p73lkq81djg.fsf@verdi.suse.de>
Date:	01 Aug 2006 21:06:27 +0200
From:	Andi Kleen <ak@...e.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	<linux-kernel@...r.kernel.org>, Horms <horms@...ge.net.au>,
	Jan Kratochvil <lace@...kratochvil.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Magnus Damm <magnus.damm@...il.com>,
	Vivek Goyal <vgoyal@...ibm.com>, Linda Wang <lwang@...hat.com>
Subject: Re: [PATCH 2/33] i386: define __pa_symbol

"Eric W. Biederman" <ebiederm@...ssion.com> 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@...ssion.com>
> ---
>  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 MAXMEM			(-__PAGE_OFFSET-__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.

-Andi
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