diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 8f2251d2a3f8..ddc81a06edb9 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -237,7 +237,25 @@ void __init xen_build_dynamic_phys_to_machine(void)
 			p2m_top[topidx] = mid;
 		}
 
-		p2m_top[topidx][mididx] = &mfn_list[pfn];
+		/*
+		 * As long as the mfn_list has enough entries to completely
+		 * fill a p2m page, pointing into the array is ok. But if
+		 * not the entries beyond the last pfn will be undefined.
+		 * And guessing that the 'what-ever-there-is' does not take it
+		 * too kindly when changing it to invalid markers, a new page
+		 * is allocated, initialized and filled with the valid part.
+		 */
+		if (unlikely(pfn + P2M_PER_PAGE > max_pfn)) {
+			unsigned long p2midx;
+			unsigned long *p2m = extend_brk(PAGE_SIZE, PAGE_SIZE);
+			p2m_init(p2m);
+
+			for (p2midx = 0; pfn + p2midx < max_pfn; p2midx++) {
+				p2m[p2midx] = mfn_list[pfn + p2midx];
+			}
+			p2m_top[topidx][mididx] = p2m;
+		} else
+			p2m_top[topidx][mididx] = &mfn_list[pfn];
 	}
 
 	m2p_override_init();