diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index ddc81a06edb9..fd12d7ce7ff9 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -241,21 +241,15 @@ void __init xen_build_dynamic_phys_to_machine(void)
 		 * 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];
+			p2midx = max_pfn % P2M_PER_PAGE;
+			for ( ; p2midx < P2M_PER_PAGE; p2midx++)
+				mfn_list[pfn + p2midx] = INVALID_P2M_ENTRY;
+		}
+		p2m_top[topidx][mididx] = &mfn_list[pfn];
 	}
 
 	m2p_override_init();
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index b5a7f928234b..a8a66a50d446 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -179,8 +179,13 @@ char * __init xen_memory_setup(void)
 	e820.nr_map = 0;
 	xen_extra_mem_start = mem_end;
 	for (i = 0; i < memmap.nr_entries; i++) {
-		unsigned long long end = map[i].addr + map[i].size;
+		unsigned long long end;
 
+		/* Guard against non-page aligned E820 entries. */
+		if (map[i].type == E820_RAM)
+			map[i].size -= (map[i].size + map[i].addr) % PAGE_SIZE;
+
+		end = map[i].addr + map[i].size;
 		if (map[i].type == E820_RAM && end > mem_end) {
 			/* RAM off the end - may be partially included */
 			u64 delta = min(map[i].size, end - mem_end);
@@ -350,6 +355,7 @@ void __init xen_arch_setup(void)
 	boot_cpu_data.hlt_works_ok = 1;
 #endif
 	pm_idle = default_idle;
+	boot_option_idle_override = IDLE_HALT;
 
 	fiddle_vdso();
 }