vmm: Store and restore virtio-pci device on right PCI slot

Based on the new field "pci_bdf", a virtio-pci device can be restored at
the same place on the PCI bus it was located before the VM snapshot.

This ensures consistent placement on the PCI bus, based on the stored
information related to each device.

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

vmm/src/device_manager.rs

@@ -286,6 +286,10 @@ pub enum DeviceManagerError {
     #[cfg(feature = "pci_support")]
     NextPciDeviceId(pci::PciRootError),
 
+    /// Could not reserve the PCI device ID.
+    #[cfg(feature = "pci_support")]
+    GetPciDeviceId(pci::PciRootError),
+
     /// Could not give the PCI device ID back.
     #[cfg(feature = "pci_support")]
     PutPciDeviceId(pci::PciRootError),
@@ -332,6 +336,10 @@ pub enum DeviceManagerError {
 
     /// Expected resources for virtio-fs could not be found.
     MissingVirtioFsResources,
+
+    /// Missing PCI b/d/f from the DeviceNode.
+    #[cfg(feature = "pci_support")]
+    MissingDeviceNodePciBdf,
 }
 pub type DeviceManagerResult<T> = result::Result<T, DeviceManagerError>;
 
@@ -2158,6 +2166,28 @@ impl DeviceManager {
         let mut node = device_node!(id);
         node.children = vec![virtio_device_id.clone()];
 
+        // Look for the id in the device tree. If it can be found, that means
+        // the device is being restored, otherwise it's created from scratch.
+        let pci_device_bdf = if let Some(node) = self.device_tree.get(&id) {
+            debug!("Restoring virtio-pci {} resources", id);
+            let pci_device_bdf = node
+                .pci_bdf
+                .ok_or(DeviceManagerError::MissingDeviceNodePciBdf)?;
+
+            pci.get_device_id((pci_device_bdf >> 3) as usize)
+                .map_err(DeviceManagerError::GetPciDeviceId)?;
+
+            pci_device_bdf
+        } else {
+            // We need to shift the device id since the 3 first bits are dedicated
+            // to the PCI function, and we know we don't do multifunction.
+            // Also, because we only support one PCI bus, the bus 0, we don't need
+            // to add anything to the global device ID.
+            pci.next_device_id()
+                .map_err(DeviceManagerError::NextPciDeviceId)?
+                << 3
+        };
+
         // Update the existing virtio node by setting the parent.
         if let Some(node) = self.device_tree.get_mut(&virtio_device_id) {
             node.parent = Some(id.clone());
@@ -2170,15 +2200,6 @@ impl DeviceManager {
         // about a virtio config change.
         let msix_num = (virtio_device.lock().unwrap().queue_max_sizes().len() + 1) as u16;
 
-        // We need to shift the device id since the 3 first bits are dedicated
-        // to the PCI function, and we know we don't do multifunction.
-        // Also, because we only support one PCI bus, the bus 0, we don't need
-        // to add anything to the global device ID.
-        let pci_device_bdf = pci
-            .next_device_id()
-            .map_err(DeviceManagerError::NextPciDeviceId)?
-            << 3;
-
         // Create the callback from the implementation of the DmaRemapping
         // trait. The point with the callback is to simplify the code as we
         // know about the device ID from this point.
@@ -2251,6 +2272,7 @@ impl DeviceManager {
         .map_err(DeviceManagerError::AddPciDevice)?;
 
         node.migratable = Some(Arc::clone(&virtio_pci_device) as Arc<Mutex<dyn Migratable>>);
+        node.pci_bdf = Some(pci_device_bdf);
         self.device_tree.insert(id, node);
 
         Ok(pci_device_bdf)