--- 2.6/include/asm-i386/io_apic.h	2005-05-23 12:45:51.000000000 -0700
+++ 2.6/include/asm-i386/io_apic.h	2005-05-23 12:45:51.000000000 -0700
@@ -196,6 +196,7 @@ extern int skip_ioapic_setup;
 #define io_apic_assign_pci_irqs (mp_irq_entries && !skip_ioapic_setup && io_apic_irqs)
 
 #ifdef CONFIG_ACPI_BOOT
+extern unsigned char io_apic_read_index (int ioapic);
 extern int io_apic_get_unique_id (int ioapic, int apic_id);
 extern int io_apic_get_version (int ioapic);
 extern int io_apic_get_redir_entries (int ioapic);
--- 2.6/arch/i386/kernel/mpparse.c	2005-05-23 13:12:35.000000000 -0700
+++ 2.6/arch/i386/kernel/mpparse.c	2005-05-23 13:12:35.000000000 -0700
@@ -907,16 +907,34 @@ void __init mp_register_ioapic (
 		return;
 	}
 
-	idx = nr_ioapics++;
+	/*
+	 * Probe the IO APIC. Set up the ioapic structures so we can do reads,
+	 * but delay incrementing nr_ioapics in case the probe fails
+	 */
+	idx = nr_ioapics;
 
 	mp_ioapics[idx].mpc_type = MP_IOAPIC;
 	mp_ioapics[idx].mpc_flags = MPC_APIC_USABLE;
 	mp_ioapics[idx].mpc_apicaddr = address;
 
 	set_fixmap_nocache(FIX_IO_APIC_BASE_0 + idx, address);
-	mp_ioapics[idx].mpc_apicid = io_apic_get_unique_id(idx, id);
+
 	mp_ioapics[idx].mpc_apicver = io_apic_get_version(idx);
-	
+	/*
+	 * Probe to see if the io_apic looks good.
+	 * index register should be set as well.
+	 */
+	if (mp_ioapics[idx].mpc_apicver == 0xff) {
+		if (io_apic_read_index(idx) == 0xff) {
+			printk(KERN_ERR "WARNING: Bogus (0x%08x) I/O APIC address"
+				" found in MADT table, skipping!\n", address);
+			return;
+		}
+	}
+	nr_ioapics++;
+
+	mp_ioapics[idx].mpc_apicid = io_apic_get_unique_id(idx, id);
+
 	/* 
 	 * Build basic GSI lookup table to facilitate gsi->io_apic lookups
 	 * and to prevent reprogramming of IOAPIC pins (PCI GSIs).
--- 2.6/arch/i386/kernel/io_apic.c	2005-05-23 12:48:32.000000000 -0700
+++ 2.6/arch/i386/kernel/io_apic.c	2005-05-23 12:48:32.000000000 -0700
@@ -2459,6 +2459,15 @@ int __init io_apic_get_unique_id (int io
 	return apic_id;
 }
 
+unsigned char __init io_apic_read_index (int ioapic)
+{
+	unsigned long flags;
+	unsigned char index_reg;
+	spin_lock_irqsave(&ioapic_lock, flags);
+	index_reg = *IO_APIC_BASE(ioapic);
+	spin_unlock_irqrestore(&ioapic_lock, flags);
+	return index_reg;
+}
 
 int __init io_apic_get_version (int ioapic)
 {