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powerpc: Work around gcc miscompilation of __pa() on 64-bit

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On 64-bit, __pa(&static_var) gets miscompiled by recent versions of
gcc as something like:

        addis 3,2,.LANCHOR1+4611686018427387904@toc@ha
        addi 3,3,.LANCHOR1+4611686018427387904@toc@l

This ends up effectively ignoring the offset, since its bottom 32 bits
are zero, and means that the result of __pa() still has 0xC in the top
nibble.  This happens with gcc 4.8.1, at least.

To work around this, for 64-bit we make __pa() use an AND operator,
and for symmetry, we make __va() use an OR operator.  Using an AND
operator rather than a subtraction ends up with slightly shorter code
since it can be done with a single clrldi instruction, whereas it
takes three instructions to form the constant (-PAGE_OFFSET) and add
it on.  (Note that MEMORY_START is always 0 on 64-bit.)

CC: <stable@vger.kernel.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This commit is contained in:
Paul Mackerras 2013-08-27 16:07:49 +10:00 committed by Benjamin Herrenschmidt
parent f5f6cbb616
commit bdbc29c19b
2 changed files with 11 additions and 0 deletions
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1
arch/powerpc/Kconfig
View File

@ -979,6 +979,7 @@ config RELOCATABLE
must live at a different physical address than the primary
kernel.
# This value must have zeroes in the bottom 60 bits otherwise lots will break
config PAGE_OFFSET
hex
default "0xc000000000000000"

10
arch/powerpc/include/asm/page.h
View File

@ -211,9 +211,19 @@ extern long long virt_phys_offset;
#define __va(x) ((void *)(unsigned long)((phys_addr_t)(x) + VIRT_PHYS_OFFSET))
#define __pa(x) ((unsigned long)(x) - VIRT_PHYS_OFFSET)
#else
#ifdef CONFIG_PPC64
/*
* gcc miscompiles (unsigned long)(&static_var) - PAGE_OFFSET
* with -mcmodel=medium, so we use & and | instead of - and + on 64-bit.
*/
#define __va(x) ((void *)(unsigned long)((phys_addr_t)(x) | PAGE_OFFSET))
#define __pa(x) ((unsigned long)(x) & 0x0fffffffffffffffUL)
#else /* 32-bit, non book E */
#define __va(x) ((void *)(unsigned long)((phys_addr_t)(x) + PAGE_OFFSET - MEMORY_START))
#define __pa(x) ((unsigned long)(x) - PAGE_OFFSET + MEMORY_START)
#endif
#endif
/*
* Unfortunately the PLT is in the BSS in the PPC32 ELF ABI,