vhost-user-backend

Design

The vhost-user-backend crate provides a framework to implement vhost-user backend services, which includes following external public APIs:

• A daemon control object (VhostUserDaemon) to start and stop the service daemon.
• A vhost-user backend trait (VhostUserBackendMut) to handle vhost-user control messages and virtio messages.
• A vring access trait (VringT) to access virtio queues, and three implementations of the trait: VringState, VringMutex and VringRwLock.

Usage

The vhost-user-backend crate provides a framework to implement vhost-user backend services. The main interface provided by vhost-user-backend library is the struct VhostUserDaemon:

pub struct VhostUserDaemon<S, V, B = ()>
where
    S: VhostUserBackend<V, B>,
    V: VringT<GM<B>> + Clone + Send + Sync + 'static,
    B: Bitmap + 'static,
{
    pub fn new(name: String, backend: S, atomic_mem: GuestMemoryAtomic<GuestMemoryMmap<B>>) -> Result<Self>;
    pub fn start(&mut self, listener: Listener) -> Result<()>;
    pub fn wait(&mut self) -> Result<()>;
    pub fn get_epoll_handlers(&self) -> Vec<Arc<VringEpollHandler<S, V, B>>>;
}

Create a VhostUserDaemon Instance

The VhostUserDaemon::new() creates an instance of VhostUserDaemon object. The client needs to pass in an VhostUserBackend object, which will be used to configure the VhostUserDaemon instance, handle control messages from the vhost-user frontend and handle virtio requests from virtio queues. A group of working threads will be created to handle virtio requests from configured virtio queues.

Start the VhostUserDaemon

The VhostUserDaemon::start() method waits for an incoming connection from the vhost-user frontends on the listener. Once a connection is ready, a main thread will be created to handle vhost-user messages from the vhost-user frontend.

Stop the VhostUserDaemon

The VhostUserDaemon::stop() method waits for the main thread to exit. An exit event must be sent to the main thread by writing to the exit_event EventFd before waiting for it to exit.

Threading Model

The main thread and virtio queue working threads will concurrently access the underlying virtio queues, so all virtio queue in multi-threading model. But the main thread only accesses virtio queues for configuration, so client could adopt locking policies to optimize for the virtio queue working threads.

Example

Example code to handle virtio messages from a virtio queue:

impl VhostUserBackendMut for VhostUserService {
    fn process_queue(&mut self, vring: &VringMutex) -> Result<bool> {
        let mut used_any = false;
        let mem = match &self.mem {
            Some(m) => m.memory(),
            None => return Err(Error::NoMemoryConfigured),
        };

        let mut vring_state = vring.get_mut();

        while let Some(avail_desc) = vring_state
            .get_queue_mut()
            .iter()
            .map_err(|_| Error::IterateQueue)?
            .next()
        {
            // Process the request...

            if self.event_idx {
                if vring_state.add_used(head_index, 0).is_err() {
                    warn!("Couldn't return used descriptors to the ring");
                }

                match vring_state.needs_notification() {
                    Err(_) => {
                        warn!("Couldn't check if queue needs to be notified");
                        vring_state.signal_used_queue().unwrap();
                    }
                    Ok(needs_notification) => {
                        if needs_notification {
                            vring_state.signal_used_queue().unwrap();
                        }
                    }
                }
            } else {
                if vring_state.add_used(head_index, 0).is_err() {
                    warn!("Couldn't return used descriptors to the ring");
                }
                vring_state.signal_used_queue().unwrap();
            }
        }

        Ok(used_any)
    }
}

Postcopy support

To enabled POSTCOPY_* messages support there is a postcopy feature. Due to how Xen handles memory mappings the postcopy feature is not compatible with xen feature. Enabling both at the same time will result in a compilation error.

postcopy feature enables optional userfaultfd dependency in order to create and interact with userfaultfd object. This requires access permission to /dev/userfaultfd file from the backend.

Xen support

Supporting Xen requires special handling while mapping the guest memory. The vm-memory crate implements xen memory mapping support via a separate feature xen, and this crate uses the same feature name to enable Xen support.

Also, for xen mappings, the memory regions passed by the frontend contains few extra fields as described in the vhost-user protocol documentation.

It was decided by the rust-vmm maintainers to keep the interface simple and build the crate for either standard Unix memory mapping or Xen, and not both.

License

This project is licensed under

• Apache License, Version 2.0