--- 2006-04-19/linux-2.6-xen-sparse/arch/i386/kernel/io_apic-xen.c.0	2006-04-20 14:18:14.000000000 +0200
+++ 2006-04-19/linux-2.6-xen-sparse/arch/i386/kernel/io_apic-xen.c	2006-04-20 14:26:17.000000000 +0200
@@ -2483,8 +2483,17 @@ __setup("enable_8254_timer", setup_enabl
  
 static int __init io_apic_bug_finalize(void)
 {
-	if(sis_apic_bug == -1)
+	if(sis_apic_bug == -1) {
+		if (xen_start_info->flags & SIF_INITDOMAIN) {
+			dom0_op_t op = {
+				.cmd = DOM0_PLATFORM_QUIRK,
+				.u.platform_quirk.quirk_id = QUIRK_IOAPICMODIFY
+			};
+
+			HYPERVISOR_dom0_op(&op);
+		}
 		sis_apic_bug = 0;
+	}
 	return 0;
 }
 
--- 2006-04-19/xen/arch/x86/dom0_ops.c.0	2006-04-06 17:50:25.000000000 +0200
+++ 2006-04-19/xen/arch/x86/dom0_ops.c	2006-04-20 14:04:15.000000000 +0200
@@ -386,6 +386,12 @@ long arch_do_dom0_op(struct dom0_op *op,
             opt_noirqbalance = 1;
             setup_ioapic_dest();
             break;
+        case QUIRK_IOAPICMODIFY:
+#ifndef sis_apic_bug
+            printk("Platform info -- Allowing fast IO-APIC updates.\n");
+            sis_apic_bug = 0;
+#endif
+            break;
         default:
             ret = -EINVAL;
             break;
--- 2006-04-19/xen/arch/x86/io_apic.c.0	2006-04-20 09:36:12.000000000 +0200
+++ 2006-04-19/xen/arch/x86/io_apic.c	2006-04-20 14:04:25.000000000 +0200
@@ -51,6 +51,14 @@ static DEFINE_SPINLOCK(vector_lock);
 
 int skip_ioapic_setup;
 
+#ifndef sis_apic_bug
+/*
+ *	Is the SiS APIC rmw bug present ?
+ *	-1 = don't know, 0 = no, 1 = yes
+ */
+int sis_apic_bug = -1;
+#endif
+
 /*
  * # of IRQ routing registers
  */
--- 2006-04-19/xen/include/public/dom0_ops.h.0	2006-04-20 09:30:23.000000000 +0200
+++ 2006-04-19/xen/include/public/dom0_ops.h	2006-04-20 13:09:17.000000000 +0200
@@ -405,6 +405,7 @@ DEFINE_GUEST_HANDLE(dom0_getdomaininfoli
 
 #define DOM0_PLATFORM_QUIRK      39
 #define QUIRK_NOIRQBALANCING  1
+#define QUIRK_IOAPICMODIFY  2
 typedef struct dom0_platform_quirk {
     /* IN variables. */
     uint32_t quirk_id;
--- 2006-04-19/xen/include/asm-x86/io_apic.h.0	2006-04-06 17:50:27.000000000 +0200
+++ 2006-04-19/xen/include/asm-x86/io_apic.h	2006-04-20 13:13:35.000000000 +0200
@@ -139,7 +139,11 @@ static inline void io_apic_write(unsigne
  *
  * Older SiS APIC requires we rewrite the index regiser
  */
-#define sis_apic_bug 0 /* This may need propagating from domain0. */
+#ifdef __i386__
+extern int sis_apic_bug;
+#else
+# define sis_apic_bug 0
+#endif
 static inline void io_apic_modify(unsigned int apic, unsigned int reg, unsigned int value)
 {
 	if (sis_apic_bug)