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

Eric W. Biederman
2006-08-02T02:20:59Z

Andi Kleen <ak@suse.de> writes:

> "Eric W. Biederman" <ebiederm@xmission.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@xmission.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.

Yes. ISO C only defines pointer arithmetic with in arrays.
I believe gnu C makes it a well defined case.

Currently we do not appear to have any problems on i386.
But I have at least one case of code that is shared between
i386 and x86_64 and it is appropriate to use __pa_symbol on
x86_64.

So I added __pa_symbol for that practical reason.

I would have no problems with generalizing this but I wanted to
at least make it possible to use the concept on i386.

I will be happy to add in the assembly magic, if you don't have
any other problems with this.

Eric