x86, NMI: Allow NMI reason io port (0x61) to be processed on any CPU

In original NMI handler, NMI reason io port (0x61) is only processed
on BSP.  This makes it impossible to hot-remove BSP.  To solve the
issue, a raw spinlock is used to allow the port to be processed on any
CPU.

Originally-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1294348732-15030-6-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/traps.c

@@ -84,6 +84,11 @@ EXPORT_SYMBOL_GPL(used_vectors);
 static int ignore_nmis;
 
 int unknown_nmi_panic;
+/*
+ * Prevent NMI reason port (0x61) being accessed simultaneously, can
+ * only be used in NMI handler.
+ */
+static DEFINE_RAW_SPINLOCK(nmi_reason_lock);
 
 static inline void conditional_sti(struct pt_regs *regs)
 {
@@ -392,7 +397,6 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
 static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
 {
 	unsigned char reason = 0;
-	int cpu;
 
 	/*
 	 * CPU-specific NMI must be processed before non-CPU-specific
@@ -402,13 +406,12 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
 	if (notify_die(DIE_NMI, "nmi", regs, 0, 2, SIGINT) == NOTIFY_STOP)
 		return;
 
-	cpu = smp_processor_id();
-
-	/* Only the BSP gets external NMIs from the system. */
-	if (!cpu)
-		reason = get_nmi_reason();
+	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
+	raw_spin_lock(&nmi_reason_lock);
+	reason = get_nmi_reason();
 
 	if (!(reason & NMI_REASON_MASK)) {
+		raw_spin_unlock(&nmi_reason_lock);
 		unknown_nmi_error(reason, regs);
 
 		return;
@@ -426,6 +429,7 @@ static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
 	 */
 	reassert_nmi();
 #endif
+	raw_spin_unlock(&nmi_reason_lock);
 }
 
 dotraplinkage notrace __kprobes void