[Xen-devel] [tip:x86/asm] x86/msr: Set the return value to zero when native_rdmsr_safe() fails

[tip-bot for Andy Lutomirski <tipbot@xxxxxxxxx>]

Commit-ID:  b828b79fcced0e66492590707649dbfaea6435e6
Gitweb:     http://git.kernel.org/tip/b828b79fcced0e66492590707649dbfaea6435e6
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Sat, 2 Apr 2016 07:01:40 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 13 Apr 2016 11:37:46 +0200

x86/msr: Set the return value to zero when native_rdmsr_safe() fails

This will cause unchecked native_rdmsr_safe() failures to return
deterministic results.

Tested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: KVM list <kvm@xxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: xen-devel <Xen-devel@xxxxxxxxxxxxx>
Link: 
http://lkml.kernel.org/r/515fb611449a755312a476cfe11675906e7ddf6c.1459605520.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/msr.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 00050c0..7dc1d8f 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -101,7 +101,10 @@ static inline unsigned long long 
native_read_msr_safe(unsigned int msr,
        asm volatile("2: rdmsr ; xor %[err],%[err]\n"
                     "1:\n\t"
                     ".section .fixup,\"ax\"\n\t"
-                    "3:  mov %[fault],%[err] ; jmp 1b\n\t"
+                    "3: mov %[fault],%[err]\n\t"
+                    "xorl %%eax, %%eax\n\t"
+                    "xorl %%edx, %%edx\n\t"
+                    "jmp 1b\n\t"
                     ".previous\n\t"
                     _ASM_EXTABLE(2b, 3b)
                     : [err] "=r" (*err), EAX_EDX_RET(val, low, high)

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