[tip:x86/urgent] x86: Document rdmsr_safe restrictions

[tip-bot for Borislav Petkov <borislav.petkov@amd.com>]

Commit-ID:  ce37defc0f6673f5ca2c92ed5cfcaf290ae7dd16
Gitweb:     http://git.kernel.org/tip/ce37defc0f6673f5ca2c92ed5cfcaf290ae7dd16
Author:     Borislav Petkov <borislav.petkov@amd.com>
AuthorDate: Mon, 5 Dec 2011 14:28:37 +0100
Committer:  Borislav Petkov <borislav.petkov@amd.com>
CommitDate: Mon, 5 Dec 2011 14:28:37 +0100

x86: Document rdmsr_safe restrictions

Recently, I got bitten by using rdmsr_safe too early in the boot
process. Document its shortcomings for future reference.

Link: http://lkml.kernel.org/r/4ED5B70F.606@lwfinger.net
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
---
 arch/x86/include/asm/msr.h |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 084ef95..95203d4 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -169,7 +169,14 @@ static inline int wrmsr_safe(unsigned msr, unsigned low, unsigned high)
 	return native_write_msr_safe(msr, low, high);
 }
 
-/* rdmsr with exception handling */
+/*
+ * rdmsr with exception handling.
+ *
+ * Please note that the exception handling works only after we've
+ * switched to the "smart" #GP handler in trap_init() which knows about
+ * exception tables - using this macro earlier than that causes machine
+ * hangs on boxes which do not implement the @msr in the first argument.
+ */
 #define rdmsr_safe(msr, p1, p2)					\
 ({								\
 	int __err;						\