diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index de23fec44a..1e9da1002b 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -5320,7 +5320,7 @@ int libxl_set_vcpuaffinity(libxl_ctx *ctx, uint32_t domid, uint32_t vcpuid,
         if (rc)
             goto out;
 
-        libxl_bitmap_copy(ctx, &hard, cpumap_hard);
+        libxl__bitmap_copy_best_effort(gc, &hard, cpumap_hard);
         flags = XEN_VCPUAFFINITY_HARD;
     }
     if (cpumap_soft) {
@@ -5328,7 +5328,7 @@ int libxl_set_vcpuaffinity(libxl_ctx *ctx, uint32_t domid, uint32_t vcpuid,
         if (rc)
             goto out;
 
-        libxl_bitmap_copy(ctx, &soft, cpumap_soft);
+        libxl__bitmap_copy_best_effort(gc, &soft, cpumap_soft);
         flags |= XEN_VCPUAFFINITY_SOFT;
     }
 
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 4361421421..a38f695751 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3617,6 +3617,17 @@ static inline void libxl__update_config_vtpm(libxl__gc *gc,
         libxl_device_##type##_copy(CTX, DA_p, (dev));           \
     })
 
+/* This function copies X bytes from source to destination bitmap,
+ * where X is the smaller of the two sizes.
+ *
+ * If destination's size is larger than source, the extra bytes are
+ * untouched.
+ *
+ * XXX This is introduced to fix a regression for 4.5. It shall
+ * be revisited in 4.6 time frame.
+ */
+void libxl__bitmap_copy_best_effort(libxl__gc *gc, libxl_bitmap *dptr,
+                                    const libxl_bitmap *sptr);
 #endif
 
 /*
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index 58df4f3b7e..3e1ba170bd 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -614,6 +614,21 @@ void libxl_bitmap_copy(libxl_ctx *ctx, libxl_bitmap *dptr,
     memcpy(dptr->map, sptr->map, sz * sizeof(*dptr->map));
 }
 
+/* This function copies X bytes from source to destination bitmap,
+ * where X is the smaller of the two sizes.
+ *
+ * If destination's size is larger than source, the extra bytes are
+ * untouched.
+ */
+void libxl__bitmap_copy_best_effort(libxl__gc *gc, libxl_bitmap *dptr,
+                                    const libxl_bitmap *sptr)
+{
+    int sz;
+
+    sz = dptr->size < sptr->size ? dptr->size : sptr->size;
+    memcpy(dptr->map, sptr->map, sz * sizeof(*dptr->map));
+}
+
 void libxl_bitmap_copy_alloc(libxl_ctx *ctx,
                              libxl_bitmap *dptr,
                              const libxl_bitmap *sptr)