diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 93e228d..83a419e 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -834,7 +834,6 @@ out:
 static void remus_failover_cb(libxl__egc *egc,
                               libxl__domain_save_state *dss, int rc);
 
-/* TODO: Explicit Checkpoint acknowledgements via recv_fd. */
 int libxl_domain_remus_start(libxl_ctx *ctx, libxl_domain_remus_info *info,
                              uint32_t domid, int send_fd, int recv_fd,
                              const libxl_asyncop_how *ao_how)
@@ -871,7 +870,7 @@ int libxl_domain_remus_start(libxl_ctx *ctx, libxl_domain_remus_info *info,
     dss->callback = remus_failover_cb;
     dss->domid = domid;
     dss->fd = send_fd;
-    /* TODO do something with recv_fd */
+    dss->recv_fd = recv_fd;
     dss->type = type;
     dss->live = 1;
     dss->debug = 0;
diff --git a/tools/libxl/libxl.h b/tools/libxl/libxl.h
index f9e3ef5..4b75d52 100644
--- a/tools/libxl/libxl.h
+++ b/tools/libxl/libxl.h
@@ -639,6 +639,15 @@ typedef struct libxl__ctx libxl_ctx;
 #define LIBXL_HAVE_DOMAIN_CREATE_RESTORE_PARAMS 1
 
 /*
+ * LIBXL_HAVE_DOMAIN_CREATE_RESTORE_SEND_BACK_FD 1
+ *
+ * If this is defined, libxl_domain_create_restore()'s API includes the
+ * send_back_fd param. This is used only with COLO, for the libxl migration
+ * back channel; other callers should pass -1.
+ */
+#define LIBXL_HAVE_DOMAIN_CREATE_RESTORE_SEND_BACK_FD 1
+
+/*
  * LIBXL_HAVE_CREATEINFO_PVH
  * If this is defined, then libxl supports creation of a PVH guest.
  */
@@ -1165,6 +1174,7 @@ int libxl_domain_create_new(libxl_ctx *ctx, libxl_domain_config *d_config,
                             LIBXL_EXTERNAL_CALLERS_ONLY;
 int libxl_domain_create_restore(libxl_ctx *ctx, libxl_domain_config *d_config,
                                 uint32_t *domid, int restore_fd,
+                                int send_back_fd,
                                 const libxl_domain_restore_params *params,
                                 const libxl_asyncop_how *ao_how,
                                 const libxl_asyncprogress_how *aop_console_how)
@@ -1185,7 +1195,7 @@ int static inline libxl_domain_create_restore_0x040200(
     libxl_domain_restore_params_init(&params);
 
     ret = libxl_domain_create_restore(
-        ctx, d_config, domid, restore_fd, &params, ao_how, aop_console_how);
+        ctx, d_config, domid, restore_fd, -1, &params, ao_how, aop_console_how);
 
     libxl_domain_restore_params_dispose(&params);
     return ret;
@@ -1193,6 +1203,23 @@ int static inline libxl_domain_create_restore_0x040200(
 
 #define libxl_domain_create_restore libxl_domain_create_restore_0x040200
 
+#elif defined(LIBXL_API_VERSION) && LIBXL_API_VERSION >= 0x040400 \
+                                 && LIBXL_API_VERSION < 0x040700
+
+int static inline libxl_domain_create_restore_0x040400(
+    libxl_ctx *ctx, libxl_domain_config *d_config,
+    uint32_t *domid, int restore_fd,
+    const libxl_domain_restore_params *params,
+    const libxl_asyncop_how *ao_how,
+    const libxl_asyncprogress_how *aop_console_how)
+    LIBXL_EXTERNAL_CALLERS_ONLY
+{
+    return libxl_domain_create_restore(ctx, d_config, domid, restore_fd,
+                                       -1, params, ao_how, aop_console_how);
+}
+
+#define libxl_domain_create_restore libxl_domain_create_restore_0x040400
+
 #endif
 
 int libxl_domain_soft_reset(libxl_ctx *ctx,
diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index f1028bc..e72f829 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -1572,7 +1572,7 @@ static void domain_create_cb(libxl__egc *egc,
                              int rc, uint32_t domid);
 
 static int do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
-                            uint32_t *domid, int restore_fd,
+                            uint32_t *domid, int restore_fd, int send_back_fd,
                             const libxl_domain_restore_params *params,
                             const libxl_asyncop_how *ao_how,
                             const libxl_asyncprogress_how *aop_console_how)
@@ -1587,6 +1587,7 @@ static int do_domain_create(libxl_ctx *ctx, libxl_domain_config *d_config,
     libxl_domain_config_init(&cdcs->dcs.guest_config_saved);
     libxl_domain_config_copy(ctx, &cdcs->dcs.guest_config_saved, d_config);
     cdcs->dcs.restore_fd = cdcs->dcs.libxc_fd = restore_fd;
+    cdcs->dcs.send_back_fd = send_back_fd;
     if (restore_fd > -1) {
         cdcs->dcs.restore_params = *params;
         rc = libxl__fd_flags_modify_save(gc, cdcs->dcs.restore_fd,
@@ -1765,18 +1766,20 @@ int libxl_domain_create_new(libxl_ctx *ctx, libxl_domain_config *d_config,
                             const libxl_asyncop_how *ao_how,
                             const libxl_asyncprogress_how *aop_console_how)
 {
-    return do_domain_create(ctx, d_config, domid, -1, NULL,
+    return do_domain_create(ctx, d_config, domid, -1, -1, NULL,
                             ao_how, aop_console_how);
 }
 
 int libxl_domain_create_restore(libxl_ctx *ctx, libxl_domain_config *d_config,
                                 uint32_t *domid, int restore_fd,
+                                int send_back_fd,
                                 const libxl_domain_restore_params *params,
                                 const libxl_asyncop_how *ao_how,
                                 const libxl_asyncprogress_how *aop_console_how)
 {
-    return do_domain_create(ctx, d_config, domid, restore_fd, params,
-                            ao_how, aop_console_how);
+    assert(send_back_fd == -1);
+    return do_domain_create(ctx, d_config, domid, restore_fd, send_back_fd,
+                            params, ao_how, aop_console_how);
 }
 
 int libxl_domain_soft_reset(libxl_ctx *ctx,
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 15b9508..5314522 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -3132,6 +3132,7 @@ struct libxl__domain_save_state {
     uint32_t domid;
     int fd;
     int fdfl; /* original flags on fd */
+    int recv_fd;
     libxl_domain_type type;
     int live;
     int debug;
@@ -3465,6 +3466,7 @@ struct libxl__domain_create_state {
     libxl_domain_config guest_config_saved; /* vanilla config */
     int restore_fd, libxc_fd;
     int restore_fdfl; /* original flags of restore_fd */
+    int send_back_fd;
     libxl_domain_restore_params restore_params;
     uint32_t domid_soft_reset;
     libxl__domain_create_cb *callback;
diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
index 990d3c9..680aeb0 100644
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -159,6 +159,7 @@ struct domain_create {
     char *extra_config; /* extra config string */
     const char *restore_file;
     int migrate_fd; /* -1 means none */
+    int send_back_fd; /* -1 means none */
     char **migration_domname_r; /* from malloc */
 };
 
@@ -2691,6 +2692,7 @@ static uint32_t create_domain(struct domain_create *dom_info)
     int config_len = 0;
     int restore_fd = -1;
     int restore_fd_to_close = -1;
+    int send_back_fd = -1;
     const libxl_asyncprogress_how *autoconnect_console_how;
     struct save_file_header hdr;
     uint32_t domid_soft_reset = INVALID_DOMID;
@@ -2708,6 +2710,7 @@ static uint32_t create_domain(struct domain_create *dom_info)
         if (migrate_fd >= 0) {
             restore_source = "<incoming migration stream>";
             restore_fd = migrate_fd;
+            send_back_fd = dom_info->send_back_fd;
         } else {
             restore_source = restore_file;
             restore_fd = open(restore_file, O_RDONLY);
@@ -2896,7 +2899,7 @@ start:
 
         ret = libxl_domain_create_restore(ctx, &d_config,
                                           &domid, restore_fd,
-                                          &params,
+                                          send_back_fd, &params,
                                           0, autoconnect_console_how);
 
         libxl_domain_restore_params_dispose(&params);
@@ -4459,6 +4462,7 @@ static void migrate_receive(int debug, int daemonize, int monitor,
     dom_info.monitor = monitor;
     dom_info.paused = 1;
     dom_info.migrate_fd = recv_fd;
+    dom_info.send_back_fd = -1;
     dom_info.migration_domname_r = &migration_domname;
     dom_info.checkpointed_stream = checkpointed;
 
@@ -4632,6 +4636,7 @@ int main_restore(int argc, char **argv)
     dom_info.config_file = config_file;
     dom_info.restore_file = checkpoint_file;
     dom_info.migrate_fd = -1;
+    dom_info.send_back_fd = -1;
     dom_info.vnc = vnc;
     dom_info.vncautopass = vncautopass;
     dom_info.console_autoconnect = console_autoconnect;
@@ -5099,6 +5104,7 @@ int main_create(int argc, char **argv)
     dom_info.quiet = quiet;
     dom_info.config_file = filename;
     dom_info.migrate_fd = -1;
+    dom_info.send_back_fd = -1;
     dom_info.vnc = vnc;
     dom_info.vncautopass = vncautopass;
     dom_info.console_autoconnect = console_autoconnect;
diff --git a/tools/ocaml/libs/xl/xenlight_stubs.c b/tools/ocaml/libs/xl/xenlight_stubs.c
index 4133527..98b52b9 100644
--- a/tools/ocaml/libs/xl/xenlight_stubs.c
+++ b/tools/ocaml/libs/xl/xenlight_stubs.c
@@ -538,7 +538,7 @@ value stub_libxl_domain_create_restore(value ctx, value domain_config, value par
 
 	caml_enter_blocking_section();
 	ret = libxl_domain_create_restore(CTX, &c_dconfig, &c_domid, restore_fd,
-		&c_params, ao_how, NULL);
+		-1, &c_params, ao_how, NULL);
 	caml_leave_blocking_section();
 
 	free(ao_how);
