x86/xen: no need to explicitly register an NMI callback

commit ea9f9274 upstream.

Remove xen_enable_nmi() to fix a 64-bit guest crash when registering
the NMI callback on Xen 3.1 and earlier.

It's not needed since the NMI callback is set by a set_trap_table
hypercall (in xen_load_idt() or xen_write_idt_entry()).

It's also broken since it only set the current VCPU's callback.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

arch/x86/xen/setup.c

@@ -556,13 +556,7 @@ void xen_enable_syscall(void)
 	}
 #endif /* CONFIG_X86_64 */
 }
-void xen_enable_nmi(void)
-{
-#ifdef CONFIG_X86_64
-	if (register_callback(CALLBACKTYPE_nmi, nmi))
-		BUG();
-#endif
-}
+
 void __init xen_arch_setup(void)
 {
 	xen_panic_handler_init();
@@ -580,7 +574,6 @@ void __init xen_arch_setup(void)
 
 	xen_enable_sysenter();
 	xen_enable_syscall();
-	xen_enable_nmi();
 #ifdef CONFIG_ACPI
 	if (!(xen_start_info->flags & SIF_INITDOMAIN)) {
 		printk(KERN_INFO "ACPI in unprivileged domain disabled\n");