x86/idt: Keep spurious entries unset in system_vectors

With commit dc20b2d5 ("x86/idt: Move interrupt gate initialization to
IDT code") non assigned system vectors are also marked as used in
'used_vectors' (now 'system_vectors') bitmap. This makes checks in
arch_show_interrupts() whether a particular system vector is allocated to
always pass and e.g. 'Hyper-V reenlightenment interrupts' entry always
shows up in /proc/interrupts.

Another side effect of having all unassigned system vectors marked as used
is that irq_matrix_debug_show() will wrongly count them among 'System'
vectors.

As it is now ensured that alloc_intr_gate() is not called after init, it is
possible to leave unused entries in 'system_vectors' unset to fix these
issues.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200428093824.1451532-4-vkuznets@redhat.com

arch/x86/kernel/idt.c

@@ -321,7 +321,11 @@ void __init idt_setup_apic_and_irq_gates(void)
 
 #ifdef CONFIG_X86_LOCAL_APIC
 	for_each_clear_bit_from(i, system_vectors, NR_VECTORS) {
-		set_bit(i, system_vectors);
+		/*
+		 * Don't set the non assigned system vectors in the
+		 * system_vectors bitmap. Otherwise they show up in
+		 * /proc/interrupts.
+		 */
 		entry = spurious_entries_start + 8 * (i - FIRST_SYSTEM_VECTOR);
 		set_intr_gate(i, entry);
 	}