diff -r 02003bee3e80 -r c9df3571e4d2 xen/arch/x86/mpparse.c
--- a/xen/arch/x86/mpparse.c	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/arch/x86/mpparse.c	Tue Jul 07 16:37:03 2009 -0700
@@ -871,7 +871,7 @@
 } mp_ioapic_routing[MAX_IO_APICS];
 
 
-static int mp_find_ioapic (
+int mp_find_ioapic (
 	int			gsi)
 {
 	int			i = 0;
@@ -887,7 +887,13 @@
 
 	return -1;
 }
-	
+
+int mp_find_pin (
+	int			ioapic,
+	int			gsi)
+{
+	return gsi - mp_ioapic_routing[ioapic].gsi_base;
+}	
 
 void __init mp_register_ioapic (
 	u8			id, 
@@ -962,7 +968,7 @@
 	ioapic = mp_find_ioapic(gsi);
 	if (ioapic < 0)
 		return;
-	pin = gsi - mp_ioapic_routing[ioapic].gsi_base;
+	pin = mp_find_pin(ioapic, gsi);
 
 	/*
 	 * TBD: This check is for faulty timer entries, where the override
@@ -1084,7 +1090,7 @@
 		return gsi;
 	}
 
-	ioapic_pin = gsi - mp_ioapic_routing[ioapic].gsi_base;
+	ioapic_pin = mp_find_pin(ioapic, gsi);
 
 	if (ioapic_renumber_irq)
 		gsi = ioapic_renumber_irq(ioapic, gsi);
diff -r 02003bee3e80 -r c9df3571e4d2 xen/arch/x86/physdev.c
--- a/xen/arch/x86/physdev.c	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/arch/x86/physdev.c	Tue Jul 07 16:37:03 2009 -0700
@@ -361,6 +361,63 @@
         break;
     }
 
+    case PHYSDEVOP_route_gsi: {
+        struct physdev_route_gsi route_gsi;
+        unsigned gsi;
+        int vector;
+        int ioapic, pin;
+        int level, active_low;
+
+        ret = -EFAULT;
+        if ( copy_from_guest(&route_gsi, arg, 1) != 0 )
+            break;
+
+        ret = -EPERM;
+        if ( !IS_PRIV(v->domain) )
+            break;
+
+        ret = xsm_assign_vector(v->domain, route_gsi.pirq);
+        if ( ret )
+            break;
+
+        irq = route_gsi.pirq;
+        ret = -EINVAL;
+        if ( (irq < 0) || (irq >= nr_irqs) )
+            break;
+
+        vector = irq_to_vector(irq);
+        if ( vector < 0 ) {
+            ret = vector;
+            break;
+        }
+
+        spin_lock(&pcidevs_lock);
+        spin_lock(&dom0->event_lock);
+        ret = map_domain_pirq(dom0, irq, vector,
+                              MAP_PIRQ_TYPE_GSI, NULL);
+        spin_unlock(&dom0->event_lock);
+        spin_unlock(&pcidevs_lock);
+
+        if ( ret < 0 )
+            break;
+
+        gsi = route_gsi.gsi;
+
+        ret = -ENOENT;
+        ioapic = mp_find_ioapic(gsi);
+        if ( ioapic < 0 )
+            break;
+
+        pin = mp_find_pin(ioapic, gsi);
+
+        level = (route_gsi.flags & XENROUTEGSI_level) != 0;
+        active_low = (route_gsi.flags & XENROUTEGSI_active_low) != 0;
+
+        ret = io_apic_set_pci_routing(ioapic, pin, irq, level, active_low);
+
+        break;
+    }
+
     case PHYSDEVOP_set_iopl: {
         struct physdev_set_iopl set_iopl;
         ret = -EFAULT;
diff -r 02003bee3e80 -r c9df3571e4d2 xen/include/asm-x86/mpspec.h
--- a/xen/include/asm-x86/mpspec.h	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/include/asm-x86/mpspec.h	Tue Jul 07 16:37:03 2009 -0700
@@ -34,6 +34,10 @@
 extern void mp_override_legacy_irq (u8 bus_irq, u8 polarity, u8 trigger, u32 gsi);
 extern void mp_config_acpi_legacy_irqs (void);
 extern int mp_register_gsi (u32 gsi, int edge_level, int active_high_low);
+
+extern int mp_find_ioapic (int gsi);
+extern int mp_find_pin (int ioapic, int gsi);
+
 #endif /* CONFIG_ACPI */
 
 #define PHYSID_ARRAY_SIZE	BITS_TO_LONGS(MAX_APICS)
diff -r 02003bee3e80 -r c9df3571e4d2 xen/include/public/physdev.h
--- a/xen/include/public/physdev.h	Thu Jun 25 18:31:10 2009 +0100
+++ b/xen/include/public/physdev.h	Tue Jul 07 16:37:03 2009 -0700
@@ -210,6 +210,31 @@
 typedef struct physdev_manage_pci_ext physdev_manage_pci_ext_t;
 DEFINE_XEN_GUEST_HANDLE(physdev_manage_pci_ext_t);
 
+/* 
+ * Allocate a vector for an irq (if necessary) and set up routing for
+ * the GSI in the appropriate IO/APIC.  May be used to update the
+ * IO/APIC routing once it has been initially set.
+ *
+ * @arg = pointer to physdev_route_gsi structure.
+ */
+#define PHYSDEVOP_route_gsi		21
+struct physdev_route_gsi {
+	/* IN */
+	uint32_t pirq;
+	uint32_t gsi;
+	uint32_t flags;		/* XENROUTEGSI_* */
+};
+
+typedef struct physdev_route_gsi physdev_route_gsi_t;
+DEFINE_XEN_GUEST_HANDLE(physdev_route_gsi_t);
+
+#define _XENROUTEGSI_level      (0)
+#define  XENROUTEGSI_level      (1U<<_XENROUTEGSI_level)
+#define _XENROUTEGSI_active_low (1)
+#define  XENROUTEGSI_active_low (1U<<_XENROUTEGSI_active_low)
+#define _XENROUTEGSI_masked     (2)
+#define  XENROUTEGSI_masked     (1U<<_XENROUTEGSI_masked)
+
 /*
  * Argument to physdev_op_compat() hypercall. Superceded by new physdev_op()
  * hypercall since 0x00030202.