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>

virtio-devices/src/vhost_user/blk.rs

@@ -417,6 +417,10 @@ impl Migratable for Blk {
         self.vu_common.dirty_log(&self.guest_memory)
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        self.vu_common.start_migration()
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         self.vu_common
             .complete_migration(self.common.kill_evt.take())

virtio-devices/src/vhost_user/fs.rs

@@ -698,6 +698,10 @@ impl Migratable for Fs {
         self.vu_common.dirty_log(&self.guest_memory)
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        self.vu_common.start_migration()
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         self.vu_common
             .complete_migration(self.common.kill_evt.take())

virtio-devices/src/vhost_user/mod.rs

@@ -451,7 +451,6 @@ impl VhostUserCommon {
         &mut self,
         guest_memory: &Option<GuestMemoryAtomic<GuestMemoryMmap>>,
     ) -> std::result::Result<(), MigratableError> {
-        self.migration_started = true;
         if let Some(vu) = &self.vu {
             if let Some(guest_memory) = guest_memory {
                 let last_ram_addr = guest_memory.memory().last_addr().raw_value();
@@ -475,7 +474,6 @@ impl VhostUserCommon {
     }
 
     pub fn stop_dirty_log(&mut self) -> std::result::Result<(), MigratableError> {
-        self.migration_started = false;
         if let Some(vu) = &self.vu {
             vu.lock().unwrap().stop_dirty_log().map_err(|e| {
                 MigratableError::StopDirtyLog(anyhow!(
@@ -509,10 +507,17 @@ impl VhostUserCommon {
         }
     }
 
+    pub fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        self.migration_started = true;
+        Ok(())
+    }
+
     pub fn complete_migration(
         &mut self,
         kill_evt: Option<EventFd>,
     ) -> std::result::Result<(), MigratableError> {
+        self.migration_started = false;
+
         // Make sure the device thread is killed in order to prevent from
         // reconnections to the socket.
         if let Some(kill_evt) = kill_evt {

virtio-devices/src/vhost_user/net.rs

@@ -488,6 +488,10 @@ impl Migratable for Net {
         self.vu_common.dirty_log(&self.guest_memory)
     }
 
+    fn start_migration(&mut self) -> std::result::Result<(), MigratableError> {
+        self.vu_common.start_migration()
+    }
+
     fn complete_migration(&mut self) -> std::result::Result<(), MigratableError> {
         self.vu_common
             .complete_migration(self.common.kill_evt.take())