diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index bd47c3d672..f2ef977963 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -52,7 +52,7 @@ typedef struct MapCacheEntry {
     hwaddr paddr_index;
     uint8_t *vaddr_base;
     unsigned long *valid_mapping;
-    uint8_t lock;
+    uint32_t lock;
 #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
     uint8_t flags;
     hwaddr size;
@@ -355,6 +355,12 @@ tryagain:
     if (lock) {
         MapCacheRev *reventry = g_malloc0(sizeof(MapCacheRev));
         entry->lock++;
+        if (entry->lock == 0) {
+            fprintf(stderr,
+                    "mapcache entry lock overflow: "TARGET_FMT_plx" -> %p\n",
+                    entry->paddr_index, entry->vaddr_base);
+            abort();
+        }
         reventry->dma = dma;
         reventry->vaddr_req = mapcache->last_entry->vaddr_base + address_offset;
         reventry->paddr_index = mapcache->last_entry->paddr_index;