vmm, virtio-devices: Restore vhost-user devices in a dedicated way

We cannot let vhost-user devices connect to the backend when the Block,
Fs or Net object is being created during a restore/migration. The reason
is we can't have two VMs (source and destination) connected to the same
backend at the same time. That's why we must delay the connection with
the vhost-user backend until the restoration is performed.

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

virtio-devices/src/vhost_user/net.rs

@@ -44,6 +44,8 @@ pub struct State {
     pub avail_features: u64,
     pub acked_features: u64,
     pub config: VirtioNetConfig,
+    pub acked_protocol_features: u64,
+    pub vu_num_queues: usize,
 }
 
 impl VersionMapped for State {}
@@ -127,9 +129,38 @@ impl Net {
         vu_cfg: VhostUserConfig,
         server: bool,
         seccomp_action: SeccompAction,
+        restoring: bool,
     ) -> Result<Net> {
         let mut num_queues = vu_cfg.num_queues;
 
+        if restoring {
+            // We need 'queue_sizes' to report a number of queues that will be
+            // enough to handle all the potential queues. Including the control
+            // queue (with +1) will guarantee that. VirtioPciDevice::new() will
+            // create the actual queues based on this information.
+            return Ok(Net {
+                id,
+                common: VirtioCommon {
+                    device_type: VirtioDeviceType::Net as u32,
+                    queue_sizes: vec![vu_cfg.queue_size; num_queues + 1],
+                    paused_sync: Some(Arc::new(Barrier::new(2))),
+                    min_queues: DEFAULT_QUEUE_NUMBER as u16,
+                    ..Default::default()
+                },
+                vu: None,
+                config: VirtioNetConfig::default(),
+                guest_memory: None,
+                acked_protocol_features: 0,
+                socket_path: vu_cfg.socket,
+                server,
+                ctrl_queue_epoll_thread: None,
+                epoll_thread: None,
+                seccomp_action,
+                vu_num_queues: num_queues,
+                migration_started: false,
+            });
+        }
+
         // Filling device and vring features VMM supports.
         let mut avail_features = 1 << VIRTIO_NET_F_CSUM
             | 1 << VIRTIO_NET_F_GUEST_CSUM
@@ -218,6 +249,8 @@ impl Net {
             avail_features: self.common.avail_features,
             acked_features: self.common.acked_features,
             config: self.config,
+            acked_protocol_features: self.acked_protocol_features,
+            vu_num_queues: self.vu_num_queues,
         }
     }
 
@@ -225,6 +258,31 @@ impl Net {
         self.common.avail_features = state.avail_features;
         self.common.acked_features = state.acked_features;
         self.config = state.config;
+        self.acked_protocol_features = state.acked_protocol_features;
+        self.vu_num_queues = state.vu_num_queues;
+
+        let mut vu = match VhostUserHandle::connect_vhost_user(
+            self.server,
+            &self.socket_path,
+            self.vu_num_queues as u64,
+            false,
+        ) {
+            Ok(r) => r,
+            Err(e) => {
+                error!("Failed connecting vhost-user backend: {:?}", e);
+                return;
+            }
+        };
+
+        if let Err(e) = vu.set_protocol_features_vhost_user(
+            self.common.acked_features,
+            self.acked_protocol_features,
+        ) {
+            error!("Failed setting up vhost-user backend: {:?}", e);
+            return;
+        }
+
+        self.vu = Some(Arc::new(Mutex::new(vu)));
     }
 }