[PATCH 02/16] migration: Do not construct JSON description if suppressed

[Peter Xu <peterx@redhat.com>]

QEMU machine has a property "suppress-vmdesc". When it is enabled, QEMU
will stop attaching JSON VM description at the end of the precopy migration
stream (postcopy is never affected because postcopy never attach that).

However even if it's suppressed by the user, the source QEMU will still
construct the JSON descriptions, which is a complete waste of CPU and
memory resources.

To avoid it, only create the JSON writer object if suppress-vmdesc is not
specified.

Luckily, vmstate_save() already supports vmdesc==NULL, so only a few spots
that are left to be prepared that vmdesc can be NULL now.

When at it, move the init / destroy of the JSON writer object to start /
end of the migration - the JSON writer object is a sub-struct of migration
state, and that looks like the only object that was dynamically allocated /
destroyed within migration process.  Make it the same as the rest objects
that migration uses.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.h |  1 +
 migration/migration.c |  9 +++++---
 migration/savevm.c    | 49 +++++++++++++++++++++++--------------------
 3 files changed, 33 insertions(+), 26 deletions(-)

diff --git a/migration/migration.h b/migration/migration.h
index 0df2a187af..3f1927f79c 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -552,6 +552,7 @@ void migration_bitmap_sync_precopy(bool last_stage);
 
 /* migration/block-dirty-bitmap.c */
 void dirty_bitmap_mig_init(void);
+bool should_send_vmdesc(void);
 
 /* migration/block-active.c */
 void migration_block_active_setup(bool active);
diff --git a/migration/migration.c b/migration/migration.c
index 2d1da917c7..bad7b39293 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1406,8 +1406,8 @@ static void migrate_fd_cleanup(MigrationState *s)
 
     g_free(s->hostname);
     s->hostname = NULL;
-    json_writer_free(s->vmdesc);
-    s->vmdesc = NULL;
+
+    g_clear_pointer(&s->vmdesc, json_writer_free);
 
     qemu_savevm_state_cleanup();
 
@@ -1681,7 +1681,10 @@ int migrate_init(MigrationState *s, Error **errp)
     s->migration_thread_running = false;
     error_free(s->error);
     s->error = NULL;
-    s->vmdesc = NULL;
+
+    if (should_send_vmdesc()) {
+        s->vmdesc = json_writer_new(false);
+    }
 
     migrate_set_state(&s->state, MIGRATION_STATUS_NONE, MIGRATION_STATUS_SETUP);
 
diff --git a/migration/savevm.c b/migration/savevm.c
index 0c4df27177..fa03a0a264 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1231,8 +1231,7 @@ void qemu_savevm_non_migratable_list(strList **reasons)
 void qemu_savevm_state_header(QEMUFile *f)
 {
     MigrationState *s = migrate_get_current();
-
-    s->vmdesc = json_writer_new(false);
+    JSONWriter *vmdesc = s->vmdesc;
 
     trace_savevm_state_header();
     qemu_put_be32(f, QEMU_VM_FILE_MAGIC);
@@ -1241,16 +1240,21 @@ void qemu_savevm_state_header(QEMUFile *f)
     if (s->send_configuration) {
         qemu_put_byte(f, QEMU_VM_CONFIGURATION);
 
-        /*
-         * This starts the main json object and is paired with the
-         * json_writer_end_object in
-         * qemu_savevm_state_complete_precopy_non_iterable
-         */
-        json_writer_start_object(s->vmdesc, NULL);
+        if (vmdesc) {
+            /*
+             * This starts the main json object and is paired with the
+             * json_writer_end_object in
+             * qemu_savevm_state_complete_precopy_non_iterable
+             */
+            json_writer_start_object(vmdesc, NULL);
+            json_writer_start_object(vmdesc, "configuration");
+        }
+
+        vmstate_save_state(f, &vmstate_configuration, &savevm_state, vmdesc);
 
-        json_writer_start_object(s->vmdesc, "configuration");
-        vmstate_save_state(f, &vmstate_configuration, &savevm_state, s->vmdesc);
-        json_writer_end_object(s->vmdesc);
+        if (vmdesc) {
+            json_writer_end_object(vmdesc);
+        }
     }
 }
 
@@ -1296,16 +1300,19 @@ int qemu_savevm_state_setup(QEMUFile *f, Error **errp)
 {
     ERRP_GUARD();
     MigrationState *ms = migrate_get_current();
+    JSONWriter *vmdesc = ms->vmdesc;
     SaveStateEntry *se;
     int ret = 0;
 
-    json_writer_int64(ms->vmdesc, "page_size", qemu_target_page_size());
-    json_writer_start_array(ms->vmdesc, "devices");
+    if (vmdesc) {
+        json_writer_int64(vmdesc, "page_size", qemu_target_page_size());
+        json_writer_start_array(vmdesc, "devices");
+    }
 
     trace_savevm_state_setup();
     QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
         if (se->vmsd && se->vmsd->early_setup) {
-            ret = vmstate_save(f, se, ms->vmdesc, errp);
+            ret = vmstate_save(f, se, vmdesc, errp);
             if (ret) {
                 migrate_set_error(ms, *errp);
                 qemu_file_set_error(f, ret);
@@ -1424,7 +1431,7 @@ int qemu_savevm_state_iterate(QEMUFile *f, bool postcopy)
     return all_finished;
 }
 
-static bool should_send_vmdesc(void)
+bool should_send_vmdesc(void)
 {
     MachineState *machine = MACHINE(qdev_get_machine());
 
@@ -1564,21 +1571,17 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
         /* Postcopy stream will still be going */
         qemu_put_byte(f, QEMU_VM_EOF);
 
-        json_writer_end_array(vmdesc);
-        json_writer_end_object(vmdesc);
-        vmdesc_len = strlen(json_writer_get(vmdesc));
+        if (vmdesc) {
+            json_writer_end_array(vmdesc);
+            json_writer_end_object(vmdesc);
+            vmdesc_len = strlen(json_writer_get(vmdesc));
 
-        if (should_send_vmdesc()) {
             qemu_put_byte(f, QEMU_VM_VMDESCRIPTION);
             qemu_put_be32(f, vmdesc_len);
             qemu_put_buffer(f, (uint8_t *)json_writer_get(vmdesc), vmdesc_len);
         }
     }
 
-    /* Free it now to detect any inconsistencies. */
-    json_writer_free(vmdesc);
-    ms->vmdesc = NULL;
-
     trace_vmstate_downtime_checkpoint("src-non-iterable-saved");
 
     return 0;
-- 
2.47.0