vm-migration: Add start_migration() to Migratable trait

In order to clearly decouple when the migration is started compared to
when the dirty logging is started, we introduce a new method to the
Migratable trait. This clarifies the semantics as we don't end up using
start_dirty_log() for identifying when the migration has been started.
And similarly, we rely on the already existing complete_migration()
method to know when the migration has been ended.

A bug was reported when running a local migration with a vhost-user-net
device in server mode. The reason was because the migration_started
variable was never set to "true", since the start_dirty_log() function
was never invoked.

Signed-off-by: lizhaoxin1 <Lxiaoyouling@163.com>
Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/device_manager.rs

@@ -4221,6 +4221,15 @@ impl Migratable for DeviceManager {
         Ok(MemoryRangeTable::new_from_tables(tables))
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        for (_, device_node) in self.device_tree.lock().unwrap().iter() {
+            if let Some(migratable) = &device_node.migratable {
+                migratable.lock().unwrap().start_migration()?;
+            }
+        }
+        Ok(())
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         for (_, device_node) in self.device_tree.lock().unwrap().iter() {
             if let Some(migratable) = &device_node.migratable {

vmm/src/lib.rs

@@ -1100,6 +1100,9 @@ impl Vmm {
             )));
         }
 
+        // Let every Migratable object know about the migration being started.
+        vm.start_migration()?;
+
         if send_data_migration.local {
             // Now pause VM
             vm.pause()?;

vmm/src/vm.rs

@@ -2675,6 +2675,11 @@ impl Migratable for Vm {
         ]))
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        self.memory_manager.lock().unwrap().start_migration()?;
+        self.device_manager.lock().unwrap().start_migration()
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         self.memory_manager.lock().unwrap().complete_migration()?;
         self.device_manager.lock().unwrap().complete_migration()