diff -r 6fd17d0dcbcd xen/arch/x86/domain_build.c
--- a/xen/arch/x86/domain_build.c	Tue Nov 27 12:49:16 2007 +0000
+++ b/xen/arch/x86/domain_build.c	Wed Nov 28 17:37:53 2007 +0100
@@ -353,6 +353,11 @@ int __init construct_dom0(
     if ( parms.pae == PAEKERN_extended_cr3 )
             set_bit(VMASST_TYPE_pae_extended_cr3, &d->vm_assist);
 
+    /* Unconditionally initialize this member here for 64bit Dom0s.
+     * In the below if (), this is overridden for 32bit Dom0s.
+     */ 
+    d->arch.physaddr_bitsize = 64;
+
     if ( UNSET_ADDR != parms.virt_hv_start_low && elf_32bit(&elf) )
     {
 #if CONFIG_PAGING_LEVELS < 4