Guests would not manage to run successfully after being migrated to a
host having sufficiently much more memory than the host they were
originally started on.

Subsequently the plan is to re-enable the changes behavior under the
control of a guest kernel announced feature flag.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4861,7 +4861,7 @@ long arch_memory_op(int op, XEN_GUEST_HA
             .max_mfn = MACH2PHYS_NR_ENTRIES - 1
         };
 
-        if ( !mem_hotplug )
+        if ( !mem_hotplug && current->domain == dom0 )
             mapping.max_mfn = max_page - 1;
         if ( copy_to_guest(arg, &mapping, 1) )
             return -EFAULT;