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x86: Document rdmsr_safe restrictions

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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>
This commit is contained in:
Borislav Petkov 2011-12-05 14:28:37 +01:00
parent bd39906397
commit ce37defc0f
1 changed files with 8 additions and 1 deletions
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arch/x86/include/asm/msr.h
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@ -169,7 +169,14 @@ static inline int wrmsr_safe(unsigned msr, unsigned low, unsigned high)
return native_write_msr_safe(msr, low, 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) \ #define rdmsr_safe(msr, p1, p2) \
({ \ ({ \
int __err; \ int __err; \