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

diff --git a/arch/x86/kernel/apic/vector.c b/arch/x86/kernel/apic/vector.c
index 3c9c7492252f827207a8638e53318835f79b48dc..9b75a70ebf90d780438d1234a7774ddd7bc98737 100644 (file)
--- a/arch/x86/kernel/apic/vector.c
+++ b/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));
        }