Re: 2.6.1-mm2: compiler warning

[Hans Ulrich Niedermann <linux-kernel@n-dimensional.de>]

Tim Cambrant <tim@cambrant.com> writes:

> On Sat, Jan 10, 2004 at 04:40:30PM +0100, Mario Vanoni wrote:
>> Compiling the kernel under 2.6.1-mm2, gcc-3.3.2
>> (same messages as under 2.6.1-rc1-mm1, re-tested),
>> 
>> arch/i386/boot/setup.S: Assembler messages:
>> arch/i386/boot/setup.S:165: Warning: value 0x37ffffff truncated to 
>> 0x37ffffff
>> 

> If you've got a fix, it would surely be included in the kernel.

Hmm... let's see...

The assembler calculates in arch/i386/boot/setup.S (with the
definition of MAXMEM from include/asm-i386/page.h):

        - 0xC0000000 - 0x08000000 - 1

This obviously is a negative number which is what the assembler
warns us about.

As there are no negative memory addresses anyway, the number we really
want is the positive number

        (1 << 32) - 0xC0000000 - 0x08000000 - 1

which is the same in modulo (1 << 32) arithmetic, of course.

Original arch/i386/boot/setup.o (subtract from 0):

0000002c <ramdisk_max>:
      2c:	ff                   	(bad)  
      2d:	ff                   	(bad)  
      2e:	ff 37                	pushl  (%edi)

Patched arch/i386/boot/setup.o (subtract from (1<<32)):

0000002c <ramdisk_max>:
      2c:	ff                   	(bad)  
      2d:	ff                   	(bad)  
      2e:	ff 37                	pushl  (%edi)

This looks OK to me so far.

However, a few issues remain:

- There probably are some nasty side effects in other places
  which I haven't thought about.

- I didn't try to boot the patched kernel yet.

- As I don't know whether the C compiler might get confused about
  that, I re-used the #ifdef in include/asm-i386/page.h.
  Subtracting UL constants from (0UL) shouldn't make a difference,
  even if subtracting from 0x100000000UL might.

- __MAXMEM_ADDRSPACE_MAX should probably be defined using a
  constant from somewhere else. (Where from?)

Regards,

Uli

--- linux-2.6.1/include/asm-i386/page.h.orig	Sat Jan 10 18:01:31 2004
+++ linux-2.6.1/include/asm-i386/page.h	Sat Jan 10 18:03:48 2004
@@ -116,14 +116,16 @@
 
 #ifdef __ASSEMBLY__
 #define __PAGE_OFFSET		(0xC0000000)
+#define __MAXMEM_ADDRSPACE_MAX	(1 << 32)
 #else
 #define __PAGE_OFFSET		(0xC0000000UL)
+#define __MAXMEM_ADDRSPACE_MAX	(0UL)
 #endif
 
 
 #define PAGE_OFFSET		((unsigned long)__PAGE_OFFSET)
 #define VMALLOC_RESERVE		((unsigned long)__VMALLOC_RESERVE)
-#define MAXMEM			(-__PAGE_OFFSET-__VMALLOC_RESERVE)
+#define MAXMEM			(__MAXMEM_ADDRSPACE_MAX-__PAGE_OFFSET-__VMALLOC_RESERVE)
 #define __pa(x)			((unsigned long)(x)-PAGE_OFFSET)
 #define __va(x)			((void *)((unsigned long)(x)+PAGE_OFFSET))
 #define pfn_to_kaddr(pfn)      __va((pfn) << PAGE_SHIFT)