diff --git a/tools/python/xen/lowlevel/xc/xc.c b/tools/python/xen/lowlevel/xc/xc.c
index 1932758090..8ed1053575 100644
--- a/tools/python/xen/lowlevel/xc/xc.c
+++ b/tools/python/xen/lowlevel/xc/xc.c
@@ -916,7 +916,7 @@ static PyObject *pyxc_hvm_build(XcObject *self,
     char *image;
     int memsize, target=-1, vcpus = 1, acpi = 0, apic = 1;
     PyObject *vcpu_avail_handle = NULL;
-    uint8_t vcpu_avail[HVM_MAX_VCPUS/8];
+    uint8_t vcpu_avail[(HVM_MAX_VCPUS + 7)/8];
 
     static char *kwd_list[] = { "domid",
                                 "memsize", "image", "target", "vcpus", 
diff --git a/xen/include/public/hvm/hvm_info_table.h b/xen/include/public/hvm/hvm_info_table.h
index adb3fb9041..bdb5995e71 100644
--- a/xen/include/public/hvm/hvm_info_table.h
+++ b/xen/include/public/hvm/hvm_info_table.h
@@ -69,7 +69,7 @@ struct hvm_info_table {
     uint32_t    high_mem_pgend;
 
     /* Bitmap of which CPUs are online at boot time. */
-    uint8_t     vcpu_online[HVM_MAX_VCPUS/8];
+    uint8_t     vcpu_online[(HVM_MAX_VCPUS + 7)/8];
 };
 
 #endif /* __XEN_PUBLIC_HVM_HVM_INFO_TABLE_H__ */