vmm: Create devices in different path if restoring the VM

In case the VM is created from scratch, the devices should be created
after the DeviceManager has been created. But this should not affect the
restore codepath, as in this case the devices should be created as part
of the restore() function.

It's necessary to perform this differentiation as the restore must go
through the following steps:
- Create the DeviceManager
- Restore the DeviceManager with the right state
- Create the devices based on the restored DeviceManager's device tree
- Restore each device based on the restored DeviceManager's device tree

That's why this patch leverages the recent split of the DeviceManager's
creation to achieve what's needed.

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

vmm/src/device_manager.rs

@@ -2855,7 +2855,12 @@ impl Snapshottable for DeviceManager {
             )));
         }
 
-        // Then restore all devices associated with the DeviceManager.
+        // Now that DeviceManager is updated with the right states, it's time
+        // to create the devices based on the configuration.
+        self.create_devices()
+            .map_err(|e| MigratableError::Restore(anyhow!("Could not create devices {:?}", e)))?;
+
+        // Finally, restore all devices associated with the DeviceManager.
         // It's important to restore devices in the right order, that's why
         // the device tree is the right way to ensure we restore a child before
         // its parent node.

vmm/src/vm.rs

@@ -334,12 +334,6 @@ impl Vm {
         )
         .map_err(Error::DeviceManager)?;
 
-        device_manager
-            .lock()
-            .unwrap()
-            .create_devices()
-            .map_err(Error::DeviceManager)?;
-
         let cpu_manager = cpu::CpuManager::new(
             &config.lock().unwrap().cpus.clone(),
             &device_manager,
@@ -392,7 +386,7 @@ impl Vm {
         )
         .map_err(Error::MemoryManager)?;
 
-        Vm::new_from_memory_manager(
+        let vm = Vm::new_from_memory_manager(
             config,
             memory_manager,
             fd,
@@ -400,7 +394,17 @@ impl Vm {
             exit_evt,
             reset_evt,
             vmm_path,
-        )
+        )?;
+
+        // The device manager must create the devices from here as it is part
+        // of the regular code path creating everything from scratch.
+        vm.device_manager
+            .lock()
+            .unwrap()
+            .create_devices()
+            .map_err(Error::DeviceManager)?;
+
+        Ok(vm)
     }
 
     pub fn new_from_snapshot(