vmm: Implement attaching vhost-user devices

Add support for attaching vhost-user devices to the VM.
The VMM now automatically suppresses the implicit RNG
device when a vhost-user RNG is configured via
krun_add_vhost_user_device(), allowing seamless
switching between the standard virtio-rng and external
vhost-user-rng backend for better isolation and flexibility.

Signed-off-by: Dorinda Bassey <dbassey@redhat.com>

Author: Dorinda Bassey

src/vmm/src/builder.rs

@@ -213,6 +213,8 @@ pub enum StartMicrovmError {
     RegisterRngDevice(device_manager::mmio::Error),
     /// Cannot initialize a MMIO Snd device or add a device to the MMIO Bus.
     RegisterSndDevice(device_manager::mmio::Error),
+    /// Cannot initialize a vhost-user device or add a device to the MMIO Bus.
+    RegisterVhostUserDevice(device_manager::mmio::Error),
     /// Cannot initialize a MMIO Vsock Device or add a device to the MMIO Bus.
     RegisterVsockDevice(device_manager::mmio::Error),
     /// Cannot attest the VM in the Secure Virtualization context.
@@ -465,6 +467,14 @@ impl Display for StartMicrovmError {
                     "Cannot initialize a MMIO Snd Device or add a device to the MMIO Bus. {err_msg}"
                 )
             }
+            RegisterVhostUserDevice(ref err) => {
+                let mut err_msg = format!("{err}");
+                err_msg = err_msg.replace('\"', "");
+                write!(
+                    f,
+                    "Cannot initialize a vhost-user device or add a device to the MMIO Bus. {err_msg}"
+                )
+            }
             RegisterVsockDevice(ref err) => {
                 let mut err_msg = format!("{err}");
                 err_msg = err_msg.replace('\"', "");
@@ -976,7 +986,29 @@ pub fn build_microvm(
     #[cfg(not(feature = "tee"))]
     attach_balloon_device(&mut vmm, event_manager, intc.clone())?;
     #[cfg(not(feature = "tee"))]
-    attach_rng_device(&mut vmm, event_manager, intc.clone())?;
+    {
+        #[cfg(feature = "vhost-user")]
+        {
+            const VIRTIO_ID_RNG: u32 = 4;
+            for device_config in &vm_resources.vhost_user_devices {
+                attach_vhost_user_device(&mut vmm, intc.clone(), device_config)?;
+            }
+
+            let has_vhost_user_rng = vm_resources
+                .vhost_user_devices
+                .iter()
+                .any(|dev| dev.device_type == VIRTIO_ID_RNG);
+
+            if !has_vhost_user_rng {
+                attach_rng_device(&mut vmm, event_manager, intc.clone())?;
+            }
+        }
+
+        #[cfg(not(feature = "vhost-user"))]
+        {
+            attach_rng_device(&mut vmm, event_manager, intc.clone())?;
+        }
+    }
     let mut console_id = 0;
     if !vm_resources.disable_implicit_console {
         attach_console_devices(
@@ -2389,6 +2421,36 @@ fn attach_rng_device(
     Ok(())
 }
 
+#[cfg(not(feature = "tee"))]
+#[cfg(feature = "vhost-user")]
+fn attach_vhost_user_device(
+    vmm: &mut Vmm,
+    intc: IrqChip,
+    device_config: &VhostUserDeviceConfig,
+) -> std::result::Result<(), StartMicrovmError> {
+    use self::StartMicrovmError::*;
+
+    let device_name = device_config
+        .name
+        .clone()
+        .unwrap_or_else(|| format!("vhost-user-{}", device_config.device_type));
+
+    let device = Arc::new(Mutex::new(
+        devices::virtio::VhostUserDevice::new(
+            &device_config.socket_path,
+            device_config.device_type,
+            device_name.clone(),
+            device_config.num_queues,
+            &device_config.queue_sizes,
+        )
+        .map_err(|e| RegisterVhostUserDevice(device_manager::mmio::Error::VhostUserDevice(e)))?,
+    ));
+
+    attach_mmio_device(vmm, device_name, intc.clone(), device).map_err(RegisterVhostUserDevice)?;
+
+    Ok(())
+}
+
 #[cfg(feature = "gpu")]
 #[allow(clippy::too_many_arguments)]
 fn attach_gpu_device(

src/vmm/src/device_manager/kvm/mmio.rs

@@ -41,6 +41,9 @@ pub enum Error {
     DeviceNotFound,
     /// Failed to update the mmio device.
     UpdateFailed,
+    /// Failed to create vhost-user device.
+    #[cfg(feature = "vhost-user")]
+    VhostUserDevice(io::Error),
 }
 
 impl fmt::Display for Error {
@@ -59,6 +62,8 @@ impl fmt::Display for Error {
             Error::RegisterIrqFd(ref e) => write!(f, "failed to register irqfd: {e}"),
             Error::DeviceNotFound => write!(f, "the device couldn't be found"),
             Error::UpdateFailed => write!(f, "failed to update the mmio device"),
+            #[cfg(feature = "vhost-user")]
+            Error::VhostUserDevice(ref e) => write!(f, "failed to create vhost-user device: {e}"),
         }
     }
 }