[tip:x86/urgent] x86: Document __phys_reloc_hide() usage in __pa_symbol()

[tip-bot for Namhyung Kim <namhyung@gmail.com>]

Commit-ID:  8fd49936a8cac246fc9ed85508556c82cd44cf68
Gitweb:     http://git.kernel.org/tip/8fd49936a8cac246fc9ed85508556c82cd44cf68
Author:     Namhyung Kim <namhyung@gmail.com>
AuthorDate: Wed, 11 Aug 2010 15:37:41 +0900
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 11 Aug 2010 08:43:49 +0200

x86: Document __phys_reloc_hide() usage in __pa_symbol()

Until all supported versions of gcc recognize
-fno-strict-overflow, we should keep the RELOC_HIDE() magic in
__pa_symbol(). Comment it.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
LKML-Reference: <1281508661-29507-1-git-send-email-namhyung@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/include/asm/page.h |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
index 625c3f0..8ca8283 100644
--- a/arch/x86/include/asm/page.h
+++ b/arch/x86/include/asm/page.h
@@ -37,6 +37,13 @@ static inline void copy_user_page(void *to, void *from, unsigned long vaddr,
 #define __pa_nodebug(x)	__phys_addr_nodebug((unsigned long)(x))
 /* __pa_symbol should be used for C visible symbols.
    This seems to be the official gcc blessed way to do such arithmetic. */
+/*
+ * We need __phys_reloc_hide() here because gcc may assume that there is no
+ * overflow during __pa() calculation and can optimize it unexpectedly.
+ * Newer versions of gcc provide -fno-strict-overflow switch to handle this
+ * case properly. Once all supported versions of gcc understand it, we can
+ * remove this Voodoo magic stuff. (i.e. once gcc3.x is deprecated)
+ */
 #define __pa_symbol(x)	__pa(__phys_reloc_hide((unsigned long)(x)))
 
 #define __va(x)			((void *)((unsigned long)(x)+PAGE_OFFSET))