[v2,20/43] migrate_start_postcopy: Command to trigger transition to postcopy

diff --git a/hmp-commands.hx b/hmp-commands.hx
index d0943b1..53362b3 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -987,6 +987,21 @@  Enable/Disable the usage of a capability @var{capability} for migration.
 ETEXI
 
     {
+        .name       = "migrate_start_postcopy",
+        .args_type  = "",
+        .params     = "",
+        .help       = "Switch migration to postcopy mode",
+        .mhandler.cmd = hmp_migrate_start_postcopy,
+    },
+
+STEXI
+@item migrate_start_postcopy
+@findex migrate_start_postcopy
+Switch in-progress migration to postcopy mode. Ignored after the end of
+migration (or once already in postcopy).
+ETEXI
+
+    {
         .name       = "client_migrate_info",
         .args_type  = "protocol:s,hostname:s,port:i?,tls-port:i?,cert-subject:s?",
         .params     = "protocol hostname port tls-port cert-subject",
diff --git a/hmp.c b/hmp.c
index 4d1838e..e4a6189 100644
--- a/hmp.c
+++ b/hmp.c
@@ -1077,6 +1077,13 @@  void hmp_migrate_set_capability(Monitor *mon, const QDict *qdict)
     }
 }
 
+void hmp_migrate_start_postcopy(Monitor *mon, const QDict *qdict)
+{
+    Error *err = NULL;
+    qmp_migrate_start_postcopy(&err);
+    hmp_handle_error(mon, &err);
+}
+
 void hmp_set_password(Monitor *mon, const QDict *qdict)
 {
     const char *protocol  = qdict_get_str(qdict, "protocol");
diff --git a/hmp.h b/hmp.h
index 4fd3c4a..4d59e6e 100644
--- a/hmp.h
+++ b/hmp.h
@@ -64,6 +64,7 @@  void hmp_migrate_set_downtime(Monitor *mon, const QDict *qdict);
 void hmp_migrate_set_speed(Monitor *mon, const QDict *qdict);
 void hmp_migrate_set_capability(Monitor *mon, const QDict *qdict);
 void hmp_migrate_set_cache_size(Monitor *mon, const QDict *qdict);
+void hmp_migrate_start_postcopy(Monitor *mon, const QDict *qdict);
 void hmp_set_password(Monitor *mon, const QDict *qdict);
 void hmp_expire_password(Monitor *mon, const QDict *qdict);
 void hmp_eject(Monitor *mon, const QDict *qdict);
diff --git a/include/migration/migration.h b/include/migration/migration.h
index bbb616e..3c6fdd8 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -101,6 +101,9 @@  struct MigrationState
     int64_t xbzrle_cache_size;
     int64_t setup_time;
     int64_t dirty_sync_count;
+
+    /* Flag set once the migration has been asked to enter postcopy */
+    volatile bool start_postcopy;
 };
 
 void process_incoming_migration(QEMUFile *f);
diff --git a/migration.c b/migration.c
index ee923e2..c473a0f 100644
--- a/migration.c
+++ b/migration.c
@@ -364,6 +364,28 @@  void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
     }
 }
 
+void qmp_migrate_start_postcopy(Error **errp)
+{
+    MigrationState *s = migrate_get_current();
+
+    if (!migrate_postcopy_ram()) {
+        error_setg(errp, "Enable postcopy with migration_set_capability before"
+                         " the start of migration");
+        return;
+    }
+
+    if (s->state == MIG_STATE_NONE) {
+        error_setg(errp, "Postcopy must be started after migration has been"
+                         " started");
+        return;
+    }
+    /*
+     * we don't error if migration has finished since that would be racy
+     * with issuing this command.
+     */
+    s->start_postcopy = true;
+}
+
 /* shared migration helpers */
 
 static void migrate_set_state(MigrationState *s, int old_state, int new_state)
diff --git a/qapi-schema.json b/qapi-schema.json
index 77e57bd..d639a78 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -538,6 +538,14 @@ 
 { 'command': 'query-migrate-capabilities', 'returns':   ['MigrationCapabilityStatus']}
 
 ##
+# @migrate-start-postcopy
+#
+# Switch migration to postcopy mode
+#
+# Since: 2.2
+{ 'command': 'migrate-start-postcopy' }
+
+##
 # @MouseInfo:
 #
 # Information about a mouse device.
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 4be4765..96eb854 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -686,6 +686,25 @@  Example:
 
 EQMP
     {
+        .name       = "migrate-start-postcopy",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_migrate_start_postcopy,
+    },
+
+SQMP
+migrate-start-postcopy
+----------------------
+
+Switch an in-progress migration to postcopy mode. Ignored after the end of
+migration (or once already in postcopy).
+
+Example:
+-> { "execute": "migrate-start-postcopy" }
+<- { "return": {} }
+
+EQMP
+
+    {
         .name       = "query-migrate-cache-size",
         .args_type  = "",
         .mhandler.cmd_new = qmp_marshal_input_query_migrate_cache_size,