--- xen/arch/x86/hvm/hvm.c	2007-04-24 12:22:28.000000000 -0400
+++ xen.new/arch/x86/hvm/hvm.c	2007-04-24 13:58:03.000000000 -0400
@@ -1049,6 +1049,10 @@
                 break;
             case HVM_PARAM_CALLBACK_IRQ:
                 hvm_set_callback_via(d, a.value);
+#if defined(__x86_64__)
+                if (hvm_guest_x86_mode(current) == 4)
+                    d->is_compat = 1;
+#endif
                 break;
             }
             d->arch.hvm_domain.params[a.index] = a.value;