x86: don't special case first IO-APIC

It has always been puzzling me why the first IO-APIC gets special cased
in two places, and finally Xen got run on a system where this breaks:

(XEN) ACPI: IOAPIC (id[0x10] address[0xfecff000] gsi_base[0])
(XEN) IOAPIC[0]: apic_id 16, version 17, address 0xfecff000, GSI 0-2
(XEN) ACPI: IOAPIC (id[0x0f] address[0xfec00000] gsi_base[3])
(XEN) IOAPIC[1]: apic_id 15, version 17, address 0xfec00000, GSI 3-38
(XEN) ACPI: IOAPIC (id[0x0e] address[0xfec01000] gsi_base[39])
(XEN) IOAPIC[2]: apic_id 14, version 17, address 0xfec01000, GSI 39-74
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 1 global_irq 4 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 0 global_irq 5 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 3 global_irq 6 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 4 global_irq 7 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 6 global_irq 9 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 7 global_irq 10 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 8 global_irq 11 low edge)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 12 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 12 global_irq 15 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 13 global_irq 16 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 14 global_irq 17 low edge)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 15 global_irq 18 dfl dfl)

i.e. all legacy IRQs (apart from the timer one, but the firmware passed
data doesn't look right for that case anyway, as both Xen and native
Linux are falling back to use the virtual wire setup for IRQ0,
apparently rather using pin 2 of the first IO-APIC) are being handled
by the second IO-APIC.

This at once eliminates the possibility of an unmasked RTE getting
written without having got a vector put in place (in
setup_IO_APIC_irqs()).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -990,18 +990,17 @@ static void __init setup_IO_APIC_irqs(vo
             else
                 add_pin_to_irq(irq, apic, pin);
 
-            if (!apic && !IO_APIC_IRQ(irq))
+            if (!IO_APIC_IRQ(irq))
                 continue;
 
-            if (IO_APIC_IRQ(irq)) {
-                vector = assign_irq_vector(irq, NULL);
-                BUG_ON(vector < 0);
-                entry.vector = vector;
-                ioapic_register_intr(irq, IOAPIC_AUTO);
+            vector = assign_irq_vector(irq, NULL);
+            BUG_ON(vector < 0);
+            entry.vector = vector;
+            ioapic_register_intr(irq, IOAPIC_AUTO);
+
+            if (platform_legacy_irq(irq))
+                disable_8259A_irq(irq_to_desc(irq));
 
-                if (!apic && platform_legacy_irq(irq))
-                    disable_8259A_irq(irq_to_desc(irq));
-            }
             desc = irq_to_desc(irq);
             SET_DEST(entry.dest.dest32, entry.dest.logical.logical_dest,
                      cpu_mask_to_apicid(desc->arch.cpu_mask));
@@ -2245,18 +2244,15 @@ unsigned apic_gsi_base(int apic);
 
 static int apic_pin_2_gsi_irq(int apic, int pin)
 {
-    int idx, irq;
+    int idx;
 
     if (apic < 0)
        return -EINVAL;
 
-    irq = apic_gsi_base(apic) + pin;
-    if (apic == 0) {
-        idx = find_irq_entry(apic, pin, mp_INT);
-        if (idx >= 0)
-            irq = pin_2_irq(idx, apic, pin);
-    }
-    return irq;
+    idx = find_irq_entry(apic, pin, mp_INT);
+
+    return idx >= 0 ? pin_2_irq(idx, apic, pin)
+                    : apic_gsi_base(apic) + pin;
 }
 
 int ioapic_guest_read(unsigned long physbase, unsigned int reg, u32 *pval)