diff -r 77d05af7dc78 xen/drivers/passthrough/amd/pci_amd_iommu.c
--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c	Mon Feb 07 09:58:11 2011 +0000
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c	Mon Feb 07 14:05:23 2011 +0100
@@ -19,6 +19,7 @@
  */
 
 #include <xen/sched.h>
+#include <xen/paging.h>
 #include <xen/pci.h>
 #include <xen/pci_regs.h>
 #include <asm/hvm/iommu.h>
@@ -186,6 +187,31 @@ static int allocate_domain_resources(str
     return 0;
 }
 
+static unsigned long get_maxpfn(struct domain *d)
+{
+    struct page_info *page;
+    unsigned long gfn, max_pfn = 0;
+
+    /* For pv & dom0, return maximum machine frame number */
+    if ( !is_hvm_domain(d) )
+        return max_page;
+
+    spin_lock(&d->page_alloc_lock);
+
+    /* Find out maximum gfn */
+    page_list_for_each ( page, &d->page_list )
+    {
+        gfn = mfn_to_gmfn(d, page_to_mfn(page));
+        if ( gfn > max_pfn )
+            max_pfn = gfn;
+    }
+
+    spin_unlock(&d->page_alloc_lock);
+
+    ASSERT(max_pfn > 0);
+    return max_pfn;
+}
+
 static int get_paging_mode(unsigned long entries)
 {
     int level = 1;
@@ -298,8 +324,9 @@ static int reassign_device( struct domai
     list_move(&pdev->domain_list, &target->arch.pdev_list);
     pdev->domain = target;
 
-    if ( target->max_pages > 0 )
-        t->paging_mode = get_paging_mode(target->max_pages);
+    /* Adjust paging mode for hvm guest.
+     * For pv and dom0, stick with get_paging_mode(max_page) */
+    t->paging_mode = get_paging_mode(get_maxpfn(target));
 
     /* IO page tables might be destroyed after pci-detach the last device
      * In this case, we have to re-allocate root table for next pci-attach.*/