migration: Add .save_prepare() handler to struct SaveVMHandlers

Add a new .save_prepare() handler to struct SaveVMHandlers. This handler
is called early, even before migration starts, and can be used by
devices to perform early checks.

Refactor migrate_init() to be able to return errors and call
.save_prepare() from there.

Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

include/migration/register.h

@@ -20,6 +20,11 @@ typedef struct SaveVMHandlers {
     /* This runs inside the iothread lock.  */
     SaveStateHandler *save_state;
 
+    /*
+     * save_prepare is called early, even before migration starts, and can be
+     * used to perform early checks.
+     */
+    int (*save_prepare)(void *opaque, Error **errp);
     void (*save_cleanup)(void *opaque);
     int (*save_live_complete_postcopy)(QEMUFile *f, void *opaque);
     int (*save_live_complete_precopy)(QEMUFile *f, void *opaque);

migration/migration.c

@@ -1392,8 +1392,15 @@ bool migration_is_active(MigrationState *s)
             s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE);
 }
 
-void migrate_init(MigrationState *s)
+int migrate_init(MigrationState *s, Error **errp)
 {
+    int ret;
+
+    ret = qemu_savevm_state_prepare(errp);
+    if (ret) {
+        return ret;
+    }
+
     /*
      * Reinitialise all migration state, except
      * parameters/capabilities that the user set, and
@@ -1432,6 +1439,8 @@ void migrate_init(MigrationState *s)
     memset(&mig_stats, 0, sizeof(mig_stats));
     memset(&compression_counters, 0, sizeof(compression_counters));
     migration_reset_vfio_bytes_transferred();
+
+    return 0;
 }
 
 int migrate_add_blocker_internal(Error *reason, Error **errp)
@@ -1641,7 +1650,9 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
         migrate_set_block_incremental(true);
     }
 
-    migrate_init(s);
+    if (migrate_init(s, errp)) {
+        return false;
+    }
 
     return true;
 }

migration/migration.h

@@ -472,7 +472,7 @@ void migrate_fd_connect(MigrationState *s, Error *error_in);
 bool migration_is_setup_or_active(int state);
 bool migration_is_running(int state);
 
-void migrate_init(MigrationState *s);
+int migrate_init(MigrationState *s, Error **errp);
 bool migration_is_blocked(Error **errp);
 /* True if outgoing migration has entered postcopy phase */
 bool migration_in_postcopy(void);

migration/savevm.c

@@ -1233,6 +1233,30 @@ bool qemu_savevm_state_guest_unplug_pending(void)
     return false;
 }
 
+int qemu_savevm_state_prepare(Error **errp)
+{
+    SaveStateEntry *se;
+    int ret;
+
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        if (!se->ops || !se->ops->save_prepare) {
+            continue;
+        }
+        if (se->ops->is_active) {
+            if (!se->ops->is_active(se->opaque)) {
+                continue;
+            }
+        }
+
+        ret = se->ops->save_prepare(se->opaque, errp);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
 void qemu_savevm_state_setup(QEMUFile *f)
 {
     MigrationState *ms = migrate_get_current();
@@ -1619,7 +1643,10 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
         return -EINVAL;
     }
 
-    migrate_init(ms);
+    ret = migrate_init(ms, errp);
+    if (ret) {
+        return ret;
+    }
     ms->to_dst_file = f;
 
     qemu_mutex_unlock_iothread();

migration/savevm.h

@@ -31,6 +31,7 @@
 
 bool qemu_savevm_state_blocked(Error **errp);
 void qemu_savevm_non_migratable_list(strList **reasons);
+int qemu_savevm_state_prepare(Error **errp);
 void qemu_savevm_state_setup(QEMUFile *f);
 bool qemu_savevm_state_guest_unplug_pending(void);
 int qemu_savevm_state_resume_prepare(MigrationState *s);