x86: suppress warning messages on IO-APIC-less systems

Each call to mp_register_gsi() so far produced two warnings (about not
being able to find the corresponding IO-APIC pin).

However, we should use the provided information for setting the ELCR
correctly (we might want to even do this when there is an IO-APIC, if
was absolutely certain that all machines really have this register
[and specifically not some other device at the two I/O ports in
question]). It is in any case questionable that we allow Dom0 to set
this register - it could particularly be the interrupt of a plug-in
serial port card that might not work due to this. The problem is that
all Dom0 kernels to date do so, hence we can't simply #GP on such an
access (which would be the result if we disallowed access to the port
as we should have done from the beginning).

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

--- a/xen/arch/x86/mpparse.c
+++ b/xen/arch/x86/mpparse.c
@@ -1034,6 +1034,21 @@ int mp_register_gsi (u32 gsi, int trigge
 		return gsi;
 #endif
 
+	if (!nr_ioapics) {
+		unsigned int port = 0x4d0 + (gsi >> 3);
+		u8 val;
+
+		if (!platform_legacy_irq(gsi))
+			return -EINVAL;
+		val = inb(port);
+		if (triggering)
+			val |= 1 << (gsi & 7);
+		else
+			val &= ~(1 << (gsi & 7));
+		outb(val, port);
+		return 0;
+	}
+
 	ioapic = mp_find_ioapic(gsi);
 	if (ioapic < 0) {
 		printk(KERN_WARNING "No IOAPIC for GSI %u\n", gsi);