UBUNTU: SAUCE: x86: Add alternative_msr_write

CVE-2018-3639 (x86)

This is a partial backport of dd84441a
  "x86/speculation: Use IBRS if available before calling into firmware"
as later patches rely on the call.
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>

arch/x86/include/asm/nospec-branch.h

@@ -217,6 +217,16 @@ static inline void vmexit_fill_RSB(void)
 #endif
 }
 
+#define alternative_msr_write(_msr, _val, _feature)		\
+	asm volatile(ALTERNATIVE("",				\
+				 "movl %[msr], %%ecx\n\t"	\
+				 "movl %[val], %%eax\n\t"	\
+				 "movl $0, %%edx\n\t"		\
+				 "wrmsr",			\
+				 _feature)			\
+		     : : [msr] "i" (_msr), [val] "i" (_val)	\
+		     : "eax", "ecx", "edx", "memory")
+
 #endif /* __ASSEMBLY__ */
 
 /*