x86/MCE: Dump MCE to dmesg if no consumers

When there are no error record consumers registered with the kernel, the
only thing that appears in dmesg is something like:

  [  300.000326] mce: [Hardware Error]: Machine check events logged

and the error records are gone. Which is seriously counterproductive.

So let's dump them to dmesg instead, in such a case.
Requested-by: NEric Morton <Eric.Morton@amd.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20161101120911.13163-4-bp@alien8.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/cpu/mcheck/mce.c

@@ -207,8 +207,12 @@ EXPORT_SYMBOL_GPL(mce_inject_log);
 
 static struct notifier_block mce_srao_nb;
 
+static atomic_t num_notifiers;
+
 void mce_register_decode_chain(struct notifier_block *nb)
 {
+	atomic_inc(&num_notifiers);
+
 	/* Ensure SRAO notifier has the highest priority in the decode chain. */
 	if (nb != &mce_srao_nb && nb->priority == INT_MAX)
 		nb->priority -= 1;
@@ -219,6 +223,8 @@ EXPORT_SYMBOL_GPL(mce_register_decode_chain);
 
 void mce_unregister_decode_chain(struct notifier_block *nb)
 {
+	atomic_dec(&num_notifiers);
+
 	atomic_notifier_chain_unregister(&x86_mce_decoder_chain, nb);
 }
 EXPORT_SYMBOL_GPL(mce_unregister_decode_chain);
@@ -270,17 +276,17 @@ struct mca_msr_regs msr_ops = {
 	.misc	= misc_reg
 };
 
-static void print_mce(struct mce *m)
+static void __print_mce(struct mce *m)
 {
-	int ret = 0;
-
-	pr_emerg(HW_ERR "CPU %d: Machine Check Exception: %Lx Bank %d: %016Lx\n",
-	       m->extcpu, m->mcgstatus, m->bank, m->status);
+	pr_emerg(HW_ERR "CPU %d: Machine Check%s: %Lx Bank %d: %016Lx\n",
+		 m->extcpu,
+		 (m->mcgstatus & MCG_STATUS_MCIP ? " Exception" : ""),
+		 m->mcgstatus, m->bank, m->status);
 
 	if (m->ip) {
 		pr_emerg(HW_ERR "RIP%s %02x:<%016Lx> ",
 			!(m->mcgstatus & MCG_STATUS_EIPV) ? " !INEXACT!" : "",
-				m->cs, m->ip);
+			m->cs, m->ip);
 
 		if (m->cs == __KERNEL_CS)
 			print_symbol("{%s}", m->ip);
@@ -308,6 +314,13 @@ static void print_mce(struct mce *m)
 	pr_emerg(HW_ERR "PROCESSOR %u:%x TIME %llu SOCKET %u APIC %x microcode %x\n",
 		m->cpuvendor, m->cpuid, m->time, m->socketid, m->apicid,
 		cpu_data(m->extcpu).microcode);
+}
+
+static void print_mce(struct mce *m)
+{
+	int ret = 0;
+
+	__print_mce(m);
 
 	/*
 	 * Print out human-readable details about the MCE error,
@@ -569,6 +582,32 @@ static struct notifier_block mce_srao_nb = {
 	.priority = INT_MAX,
 };
 
+static int mce_default_notifier(struct notifier_block *nb, unsigned long val,
+				void *data)
+{
+	struct mce *m = (struct mce *)data;
+
+	if (!m)
+		return NOTIFY_DONE;
+
+	/*
+	 * Run the default notifier if we have only the SRAO
+	 * notifier and us registered.
+	 */
+	if (atomic_read(&num_notifiers) > 2)
+		return NOTIFY_DONE;
+
+	__print_mce(m);
+
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block mce_default_nb = {
+	.notifier_call	= mce_default_notifier,
+	/* lowest prio, we want it to run last. */
+	.priority	= 0,
+};
+
 /*
  * Read ADDR and MISC registers.
  */
@@ -2138,6 +2177,7 @@ int __init mcheck_init(void)
 {
 	mcheck_intel_therm_init();
 	mce_register_decode_chain(&mce_srao_nb);
+	mce_register_decode_chain(&mce_default_nb);
 	mcheck_vendor_init_severity();
 
 	INIT_WORK(&mce_work, mce_process_work);