x86/apic: Avoid open coded x2apic detection

enable_IR_x2apic() grew a open coded x2apic detection. Implement a
proper helper function which shares the code with the already existing
x2apic_enabled().

Made it use rdmsrl_safe as suggested by Boris.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20150115211702.285038186@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/apic.h

@@ -108,6 +108,15 @@ extern u64 native_apic_icr_read(void);
 
 extern int x2apic_mode;
 
+static inline bool apic_is_x2apic_enabled(void)
+{
+	u64 msr;
+
+	if (rdmsrl_safe(MSR_IA32_APICBASE, &msr))
+		return false;
+	return msr & X2APIC_ENABLE;
+}
+
 #ifdef CONFIG_X86_X2APIC
 /*
  * Make previous memory operations globally visible before
@@ -175,15 +184,7 @@ extern void check_x2apic(void);
 extern void enable_x2apic(void);
 static inline int x2apic_enabled(void)
 {
-	u64 msr;
-
-	if (!cpu_has_x2apic)
-		return 0;
-
-	rdmsrl(MSR_IA32_APICBASE, msr);
-	if (msr & X2APIC_ENABLE)
-		return 1;
-	return 0;
+	return cpu_has_x2apic && apic_is_x2apic_enabled();
 }
 
 #define x2apic_supported()	(cpu_has_x2apic)

arch/x86/kernel/apic/apic.c

@@ -1625,10 +1625,7 @@ void __init enable_IR_x2apic(void)
 	int ret, ir_stat;
 
 	if (!IS_ENABLED(CONFIG_X86_X2APIC)) {
-		u64 msr;
-
-		rdmsrl(MSR_IA32_APICBASE, msr);
-		if (msr & X2APIC_ENABLE)
+		if (apic_is_x2apic_enabled())
 			panic("BIOS has enabled x2apic but kernel doesn't support x2apic, please disable x2apic in BIOS.\n");
 	}