x86/vector: Add a sanity check to prevent IRQ2 allocations

To prevent another incidental removal of the IRQ2 ignore logic in the
IO/APIC code going unnoticed add a sanity check. Add some commentry at the
other place which ignores IRQ2 while at it.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210318192819.795280387@linutronix.de

arch/x86/kernel/apic/vector.c

@@ -543,6 +543,14 @@ static int x86_vector_alloc_irqs(struct irq_domain *domain, unsigned int virq,
 	if ((info->flags & X86_IRQ_ALLOC_CONTIGUOUS_VECTORS) && nr_irqs > 1)
 		return -ENOSYS;
 
+	/*
+	 * Catch any attempt to touch the cascade interrupt on a PIC
+	 * equipped system.
+	 */
+	if (WARN_ON_ONCE(info->flags & X86_IRQ_ALLOC_LEGACY &&
+			 virq == PIC_CASCADE_IR))
+		return -EINVAL;
+
 	for (i = 0; i < nr_irqs; i++) {
 		irqd = irq_domain_get_irq_data(domain, virq + i);
 		BUG_ON(!irqd);
@@ -745,6 +753,11 @@ void __init lapic_assign_system_vectors(void)
 
 	/* Mark the preallocated legacy interrupts */
 	for (i = 0; i < nr_legacy_irqs(); i++) {
+		/*
+		 * Don't touch the cascade interrupt. It's unusable
+		 * on PIC equipped machines. See the large comment
+		 * in the IO/APIC code.
+		 */
 		if (i != PIC_CASCADE_IR)
 			irq_matrix_assign(vector_matrix, ISA_IRQ_VECTOR(i));
 	}