diff --git a/arch/x86/include/asm/acpi.h b/arch/x86/include/asm/acpi.h
index 0ab4f9fd268764114e3f252b07895c4bcaf63f90..3a45668f6dc38312bc9f5761214f076a144d00d4 100644
--- a/arch/x86/include/asm/acpi.h
+++ b/arch/x86/include/asm/acpi.h
@@ -50,6 +50,7 @@ void acpi_pic_sci_set_trigger(unsigned int, u16);
 
 extern int (*__acpi_register_gsi)(struct device *dev, u32 gsi,
 				  int trigger, int polarity);
+extern void (*__acpi_unregister_gsi)(u32 gsi);
 
 static inline void disable_acpi(void)
 {
diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
index 6e5e89c3c6443776a51137293897008d2c20ff03..9098d880c476cf842598d32805ed25ac37fc9ae1 100644
--- a/arch/x86/pci/xen.c
+++ b/arch/x86/pci/xen.c
@@ -458,6 +458,7 @@ int __init pci_xen_hvm_init(void)
 	 * just how GSIs get registered.
 	 */
 	__acpi_register_gsi = acpi_register_gsi_xen_hvm;
+	__acpi_unregister_gsi = NULL;
 #endif
 
 #ifdef CONFIG_PCI_MSI
@@ -482,6 +483,7 @@ int __init pci_xen_initial_domain(void)
 	pci_msi_ignore_mask = 1;
 #endif
 	__acpi_register_gsi = acpi_register_gsi_xen;
+	__acpi_unregister_gsi = NULL;
 	/* Pre-allocate legacy irqs */
 	for (irq = 0; irq < nr_legacy_irqs(); irq++) {
 		int trigger, polarity;