migration: Do not construct JSON description if suppressed

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>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-3-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

diff --git a/migration/migration.c b/migration/migration.c
index 88b09914ecef62f2f8ec59b01d2d9c9bd0658a0c..5c335cc30b13d9eedd9c599c710915a9ff677364 100644 (file)
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1431,8 +1431,8 @@ 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();
     cpr_state_close();
@@ -1722,7 +1722,10 @@ 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/migration.h b/migration/migration.h
index fb1b8f99d31526d1dc60a5c20f71c2ee5e0e676b..4c1fafc2b5a399a34ae261f915704c0d14773492 100644 (file)
--- 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/savevm.c b/migration/savevm.c
index b8859d367fc326b35be5d6abd2ecfec2ba3d3a2a..cfe9dfaf5c9990d76f4b3ae879bb0d9c4552de3e 100644 (file)
--- 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;