x86: add unknown_nmi_panic kernel parameter

It's not possible to enable the unknown_nmi_panic sysctl option
until init is run. It's useful to be able to panic the kernel
during boot too, this adds a parameter to enable this option.
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -2158,6 +2158,10 @@ and is between 256 and 4096 characters. It is defined in the file
 			Note that genuine overcurrent events won't be
 			reported either.
 
+	unknown_nmi_panic
+			[X86-32,X86-64]
+			Set unknown_nmi_panic=1 early on boot.
+
 	usbcore.autosuspend=
 			[USB] The autosuspend time delay (in seconds) used
 			for newly-detected USB devices (default 2).  This

arch/x86/kernel/nmi.c

@@ -448,6 +448,13 @@ nmi_watchdog_tick(struct pt_regs *regs, unsigned reason)
 
 #ifdef CONFIG_SYSCTL
 
+static int __init setup_unknown_nmi_panic(char *str)
+{
+	unknown_nmi_panic = 1;
+	return 1;
+}
+__setup("unknown_nmi_panic", setup_unknown_nmi_panic);
+
 static int unknown_nmi_panic_callback(struct pt_regs *regs, int cpu)
 {
 	unsigned char reason = get_nmi_reason();