From ce37defc0f6673f5ca2c92ed5cfcaf290ae7dd16 Mon Sep 17 00:00:00 2001
From: Borislav Petkov <borislav.petkov@amd.com>
Date: Mon, 5 Dec 2011 14:28:37 +0100
Subject: [PATCH] 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 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 084ef95274cd..95203d40ffdd 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; \