[Qemu-devel] [PATCH v3 24/47] migrate_start_postcopy: Command to trigger transition to postcopy

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Once postcopy is enabled (with migrate_set_capability), the migration
will still start on precopy mode.  To cause a transition into postcopy
the:

  migrate_start_postcopy

command must be issued.  Postcopy will start sometime after this
(when it's next checked in the migration loop).

Issuing the command before migration has started will error,
and issuing after it has finished is ignored.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 hmp-commands.hx               | 15 +++++++++++++++
 hmp.c                         |  7 +++++++
 hmp.h                         |  1 +
 include/migration/migration.h |  3 +++
 migration.c                   | 22 ++++++++++++++++++++++
 qapi-schema.json              |  8 ++++++++
 qmp-commands.hx               | 19 +++++++++++++++++++
 7 files changed, 75 insertions(+)

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 3aeae47..b74121e 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -100,6 +100,9 @@ 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 6c6ec03..fb758d6 100644
--- a/migration.c
+++ b/migration.c
@@ -362,6 +362,28 @@ 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,
-- 
1.9.3