vhost_user_backend: Fix remaining issues

This commit fixes all the remaining issues that were found as part of
the integration with vhost-user-net.

It fixes the way to notify that a vring is used, by using the proper
EventFd.

It removes the process_queue() function from the trait, since the
complexity it was introducing was leading to deadlocks with mutexes.

It moves the register/unregister functions for registering custom events
from the backend, from the VringEpollHandler to the VringWorker. This
allows for a lot of simplification and solve a deadlock issue.

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

vhost_user_backend/src/lib.rs

@@ -22,7 +22,7 @@ use vhost_rs::vhost_user::{
 };
 use vm_memory::guest_memory::FileOffset;
 use vm_memory::{GuestAddress, GuestMemoryMmap};
-use vm_virtio::{DescriptorChain, Queue};
+use vm_virtio::Queue;
 use vmm_sys_util::eventfd::EventFd;
 
 #[derive(Debug)]
@@ -63,6 +63,9 @@ pub trait VhostUserBackend: Send + Sync + 'static {
     /// Virtio features.
     fn features(&self) -> u64;
 
+    /// Update guest memory regions.
+    fn update_memory(&mut self, mem: GuestMemoryMmap) -> result::Result<(), io::Error>;
+
     /// This function gets called if the backend registered some additional
     /// listeners onto specific file descriptors. The library can handle
     /// virtqueues on its own, but does not know what to do with events
@@ -71,28 +74,20 @@ pub trait VhostUserBackend: Send + Sync + 'static {
         &mut self,
         device_event: u16,
         evset: epoll::Events,
+        vrings: &Vec<Arc<RwLock<Vring>>>,
     ) -> result::Result<bool, io::Error>;
 
-    /// This function is responsible for the actual processing that needs to
-    /// happen when one of the virtqueues is available.
-    fn process_queue(
-        &mut self,
-        q_idx: u16,
-        avail_desc: &DescriptorChain,
-        mem: &GuestMemoryMmap,
-    ) -> result::Result<u32, io::Error>;
-
     /// Get virtio device configuration.
     /// A default implementation is provided as we cannot expect all backends
     /// to implement this function.
-    fn get_config(&self, offset: u32, size: u32) -> Vec<u8> {
+    fn get_config(&self, _offset: u32, _size: u32) -> Vec<u8> {
         Vec::new()
     }
 
     /// Set virtio device configuration.
     /// A default implementation is provided as we cannot expect all backends
     /// to implement this function.
-    fn set_config(&mut self, offset: u32, buf: &[u8]) -> result::Result<(), io::Error> {
+    fn set_config(&mut self, _offset: u32, _buf: &[u8]) -> result::Result<(), io::Error> {
         Ok(())
     }
 }
@@ -132,7 +127,7 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
     /// disconnects.
     pub fn start(&mut self) -> Result<()> {
         let mut slave_listener =
-            SlaveListener::new(self.sock_path.as_str(), false, self.handler.clone())
+            SlaveListener::new(self.sock_path.as_str(), true, self.handler.clone())
                 .map_err(Error::CreateSlaveListener)?;
         let mut slave_handler = slave_listener
             .accept()
@@ -161,11 +156,11 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
         Ok(())
     }
 
-    /// Retrieve the vring handler. This is necessary to perform further
+    /// Retrieve the vring worker. This is necessary to perform further
     /// actions like registering and unregistering some extra event file
-    /// descriptors, as well as forcing some vring to be processed.
-    pub fn get_vring_handler(&self) -> Arc<RwLock<VringEpollHandler<S>>> {
-        self.handler.lock().unwrap().get_vring_handler()
+    /// descriptors.
+    pub fn get_vring_worker(&self) -> Arc<VringWorker> {
+        self.handler.lock().unwrap().get_vring_worker()
     }
 }
 
@@ -179,7 +174,7 @@ struct Memory {
     mappings: Vec<AddrMapping>,
 }
 
-struct Vring {
+pub struct Vring {
     queue: Queue,
     kick: Option<EventFd>,
     call: Option<EventFd>,
@@ -197,6 +192,18 @@ impl Vring {
             enabled: false,
         }
     }
+
+    pub fn mut_queue(&mut self) -> &mut Queue {
+        &mut self.queue
+    }
+
+    pub fn signal_used_queue(&self) -> result::Result<(), io::Error> {
+        if let Some(call) = self.call.as_ref() {
+            return call.write(1);
+        }
+
+        Ok(())
+    }
 }
 
 #[derive(Debug)]
@@ -210,143 +217,104 @@ pub enum VringEpollHandlerError {
     HandleEventReadKick(io::Error),
     /// Failed to handle the event from the backend.
     HandleEventBackendHandling(io::Error),
-    /// Failed to register vring listener.
-    RegisterVringListener(io::Error),
-    /// Failed to unregister vring listener.
-    UnregisterVringListener(io::Error),
 }
 
-impl std::fmt::Display for VringEpollHandlerError {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        match self {
-            VringEpollHandlerError::ProcessQueueBackendProcessing(e) => {
-                write!(f, "failed processing queue from backend: {}", e)
-            }
-            VringEpollHandlerError::SignalUsedQueue(e) => {
-                write!(f, "failed signalling used queue: {}", e)
-            }
-            VringEpollHandlerError::HandleEventReadKick(e) => {
-                write!(f, "failed reading from kick eventfd: {}", e)
-            }
-            VringEpollHandlerError::HandleEventBackendHandling(e) => {
-                write!(f, "failed handling event from backend: {}", e)
-            }
-            VringEpollHandlerError::RegisterVringListener(e) => {
-                write!(f, "failed registering vring listener: {}", e)
-            }
-            VringEpollHandlerError::UnregisterVringListener(e) => {
-                write!(f, "failed unregistering vring listener: {}", e)
-            }
-        }
-    }
-}
-
-impl error::Error for VringEpollHandlerError {}
-
 /// Result of vring epoll handler operations.
 type VringEpollHandlerResult<T> = std::result::Result<T, VringEpollHandlerError>;
 
-pub struct VringEpollHandler<S: VhostUserBackend> {
+struct VringEpollHandler<S: VhostUserBackend> {
     backend: Arc<RwLock<S>>,
     vrings: Vec<Arc<RwLock<Vring>>>,
-    mem: Option<GuestMemoryMmap>,
-    epoll_fd: RawFd,
 }
 
 impl<S: VhostUserBackend> VringEpollHandler<S> {
-    fn update_memory(&mut self, mem: Option<GuestMemoryMmap>) {
-        self.mem = mem;
-    }
-
-    /// Trigger the processing of a virtqueue. This function is meant to be
-    /// used by the caller whenever it might need some available queues to
-    /// send data back to the guest.
-    /// A concrete example is a backend registering one extra listener for
-    /// data that needs to be sent to the guest. When the associated event
-    /// is triggered, the backend will be invoked through its `handle_event`
-    /// implementation. And in this case, the way to handle the event is to
-    /// call into `process_queue` to let it invoke the backend implementation
-    /// of `process_queue`. With this twisted trick, all common parts related
-    /// to the virtqueues can remain part of the library.
-    pub fn process_queue(&mut self, q_idx: u16) -> VringEpollHandlerResult<()> {
-        let vring = &mut self.vrings[q_idx as usize].write().unwrap();
-        let mut used_desc_heads = vec![(0, 0); vring.queue.size as usize];
-        let mut used_count = 0;
-        if let Some(mem) = &self.mem {
-            for avail_desc in vring.queue.iter(&mem) {
-                let used_len = self
-                    .backend
-                    .write()
-                    .unwrap()
-                    .process_queue(q_idx, &avail_desc, &mem)
-                    .map_err(VringEpollHandlerError::ProcessQueueBackendProcessing)?;
-
-                used_desc_heads[used_count] = (avail_desc.index, used_len);
-                used_count += 1;
-            }
-
-            for &(desc_index, len) in &used_desc_heads[..used_count] {
-                vring.queue.add_used(&mem, desc_index, len);
-            }
-        }
-
-        if used_count > 0 {
-            if let Some(call) = &vring.call {
-                call.write(1)
-                    .map_err(VringEpollHandlerError::SignalUsedQueue)?;
-            }
-        }
-
-        Ok(())
-    }
-
     fn handle_event(
-        &mut self,
+        &self,
         device_event: u16,
         evset: epoll::Events,
     ) -> VringEpollHandlerResult<bool> {
         let num_queues = self.vrings.len();
-        match device_event as usize {
-            x if x < num_queues => {
-                if let Some(kick) = &self.vrings[device_event as usize].read().unwrap().kick {
-                    kick.read()
-                        .map_err(VringEpollHandlerError::HandleEventReadKick)?;
-                }
-
-                // If the vring is not enabled, it should not be processed.
-                // The event is only read to be discarded.
-                if !self.vrings[device_event as usize].read().unwrap().enabled {
-                    return Ok(false);
-                }
-
-                self.process_queue(device_event)?;
-                Ok(false)
+        if (device_event as usize) < num_queues {
+            if let Some(kick) = &self.vrings[device_event as usize].read().unwrap().kick {
+                kick.read()
+                    .map_err(VringEpollHandlerError::HandleEventReadKick)?;
             }
-            _ => self
-                .backend
-                .write()
-                .unwrap()
-                .handle_event(device_event, evset)
-                .map_err(VringEpollHandlerError::HandleEventBackendHandling),
-        }
-    }
 
-    fn register_vring_listener(&self, q_idx: usize) -> VringEpollHandlerResult<()> {
-        if let Some(fd) = &self.vrings[q_idx].read().unwrap().kick {
-            self.register_listener(fd.as_raw_fd(), epoll::Events::EPOLLIN, q_idx as u64)
-                .map_err(VringEpollHandlerError::RegisterVringListener)
-        } else {
-            Ok(())
+            // If the vring is not enabled, it should not be processed.
+            // The event is only read to be discarded.
+            if !self.vrings[device_event as usize].read().unwrap().enabled {
+                return Ok(false);
+            }
         }
-    }
 
-    fn unregister_vring_listener(&self, q_idx: usize) -> VringEpollHandlerResult<()> {
-        if let Some(fd) = &self.vrings[q_idx].read().unwrap().kick {
-            self.unregister_listener(fd.as_raw_fd(), epoll::Events::EPOLLIN, q_idx as u64)
-                .map_err(VringEpollHandlerError::UnregisterVringListener)
-        } else {
-            Ok(())
+        self.backend
+            .write()
+            .unwrap()
+            .handle_event(device_event, evset, &self.vrings)
+            .map_err(VringEpollHandlerError::HandleEventBackendHandling)
+    }
+}
+
+#[derive(Debug)]
+/// Errors related to vring worker.
+enum VringWorkerError {
+    /// Failed while waiting for events.
+    EpollWait(io::Error),
+}
+
+/// Result of vring worker operations.
+type VringWorkerResult<T> = std::result::Result<T, VringWorkerError>;
+
+pub struct VringWorker {
+    epoll_fd: RawFd,
+}
+
+impl VringWorker {
+    fn run<S: VhostUserBackend>(&self, handler: VringEpollHandler<S>) -> VringWorkerResult<()> {
+        const EPOLL_EVENTS_LEN: usize = 100;
+        let mut events = vec![epoll::Event::new(epoll::Events::empty(), 0); EPOLL_EVENTS_LEN];
+
+        'epoll: loop {
+            let num_events = match epoll::wait(self.epoll_fd, -1, &mut events[..]) {
+                Ok(res) => res,
+                Err(e) => {
+                    if e.kind() == io::ErrorKind::Interrupted {
+                        // It's well defined from the epoll_wait() syscall
+                        // documentation that the epoll loop can be interrupted
+                        // before any of the requested events occurred or the
+                        // timeout expired. In both those cases, epoll_wait()
+                        // returns an error of type EINTR, but this should not
+                        // be considered as a regular error. Instead it is more
+                        // appropriate to retry, by calling into epoll_wait().
+                        continue;
+                    }
+                    return Err(VringWorkerError::EpollWait(e));
+                }
+            };
+
+            for event in events.iter().take(num_events) {
+                let evset = match epoll::Events::from_bits(event.events) {
+                    Some(evset) => evset,
+                    None => {
+                        let evbits = event.events;
+                        println!("epoll: ignoring unknown event set: 0x{:x}", evbits);
+                        continue;
+                    }
+                };
+
+                let ev_type = event.data as u16;
+
+                if let Err(e) = handler.handle_event(ev_type, evset) {
+                    println!(
+                        "vring handler handle event {} with error {:?}\n",
+                        ev_type, e
+                    );
+                    break 'epoll;
+                }
+            }
         }
+
+        Ok(())
     }
 
     /// Register a custom event only meaningful to the caller. When this event
@@ -387,71 +355,6 @@ impl<S: VhostUserBackend> VringEpollHandler<S> {
     }
 }
 
-#[derive(Debug)]
-/// Errors related to vring worker.
-enum VringWorkerError {
-    /// Failed while waiting for events.
-    EpollWait(io::Error),
-    /// Failed to handle event.
-    HandleEvent(VringEpollHandlerError),
-}
-
-/// Result of vring worker operations.
-type VringWorkerResult<T> = std::result::Result<T, VringWorkerError>;
-
-struct VringWorker<S: VhostUserBackend> {
-    handler: Arc<RwLock<VringEpollHandler<S>>>,
-}
-
-impl<S: VhostUserBackend> VringWorker<S> {
-    fn run(&self, epoll_fd: RawFd) -> VringWorkerResult<()> {
-        const EPOLL_EVENTS_LEN: usize = 100;
-        let mut events = vec![epoll::Event::new(epoll::Events::empty(), 0); EPOLL_EVENTS_LEN];
-
-        'epoll: loop {
-            let num_events = match epoll::wait(epoll_fd, -1, &mut events[..]) {
-                Ok(res) => res,
-                Err(e) => {
-                    if e.kind() == io::ErrorKind::Interrupted {
-                        // It's well defined from the epoll_wait() syscall
-                        // documentation that the epoll loop can be interrupted
-                        // before any of the requested events occurred or the
-                        // timeout expired. In both those cases, epoll_wait()
-                        // returns an error of type EINTR, but this should not
-                        // be considered as a regular error. Instead it is more
-                        // appropriate to retry, by calling into epoll_wait().
-                        continue;
-                    }
-                    return Err(VringWorkerError::EpollWait(e));
-                }
-            };
-
-            for event in events.iter().take(num_events) {
-                let evset = match epoll::Events::from_bits(event.events) {
-                    Some(evset) => evset,
-                    None => {
-                        let evbits = event.events;
-                        println!("epoll: ignoring unknown event set: 0x{:x}", evbits);
-                        continue;
-                    }
-                };
-
-                let ev_type = event.data as u16;
-
-                if let Err(e) = self.handler.write().unwrap().handle_event(ev_type, evset) {
-                    println!(
-                        "vring handler handle event {} with error {:?}\n",
-                        ev_type, e
-                    );
-                    break 'epoll;
-                }
-            }
-        }
-
-        Ok(())
-    }
-}
-
 #[derive(Debug)]
 /// Errors related to vhost-user handler.
 pub enum VhostUserHandlerError {
@@ -482,7 +385,7 @@ type VhostUserHandlerResult<T> = std::result::Result<T, VhostUserHandlerError>;
 
 struct VhostUserHandler<S: VhostUserBackend> {
     backend: Arc<RwLock<S>>,
-    vring_handler: Arc<RwLock<VringEpollHandler<S>>>,
+    worker: Arc<VringWorker>,
     owned: bool,
     features_acked: bool,
     acked_features: u64,
@@ -498,28 +401,30 @@ impl<S: VhostUserBackend> VhostUserHandler<S> {
         let num_queues = backend.read().unwrap().num_queues();
         let max_queue_size = backend.read().unwrap().max_queue_size();
 
-        let vrings = vec![Arc::new(RwLock::new(Vring::new(max_queue_size as u16))); num_queues];
+        let mut vrings: Vec<Arc<RwLock<Vring>>> = Vec::new();
+        for _ in 0..num_queues {
+            let vring = Arc::new(RwLock::new(Vring::new(max_queue_size as u16)));
+            vrings.push(vring);
+        }
+
         // Create the epoll file descriptor
         let epoll_fd = epoll::create(true).map_err(VhostUserHandlerError::EpollCreateFd)?;
 
-        let vring_handler = Arc::new(RwLock::new(VringEpollHandler {
+        let vring_handler = VringEpollHandler {
             backend: backend.clone(),
             vrings: vrings.clone(),
-            mem: None,
-            epoll_fd,
-        }));
-        let worker = VringWorker {
-            handler: vring_handler.clone(),
         };
+        let vring_worker = Arc::new(VringWorker { epoll_fd });
+        let worker = vring_worker.clone();
 
         thread::Builder::new()
             .name("vring_worker".to_string())
-            .spawn(move || worker.run(epoll_fd))
+            .spawn(move || vring_worker.run(vring_handler))
             .map_err(VhostUserHandlerError::SpawnVringWorker)?;
 
         Ok(VhostUserHandler {
             backend,
-            vring_handler,
+            worker,
             owned: false,
             features_acked: false,
             acked_features: 0,
@@ -531,8 +436,8 @@ impl<S: VhostUserBackend> VhostUserHandler<S> {
         })
     }
 
-    fn get_vring_handler(&self) -> Arc<RwLock<VringEpollHandler<S>>> {
-        self.vring_handler.clone()
+    fn get_vring_worker(&self) -> Arc<VringWorker> {
+        self.worker.clone()
     }
 
     fn vmm_va_to_gpa(&self, vmm_va: u64) -> VhostUserHandlerResult<u64> {
@@ -634,7 +539,13 @@ impl<S: VhostUserBackend> VhostUserSlaveReqHandler for VhostUserHandler<S> {
         let mem = GuestMemoryMmap::with_files(regions).map_err(|e| {
             VhostUserError::ReqHandlerError(io::Error::new(io::ErrorKind::Other, e))
         })?;
-        self.vring_handler.write().unwrap().update_memory(Some(mem));
+        self.backend
+            .write()
+            .unwrap()
+            .update_memory(mem)
+            .map_err(|e| {
+                VhostUserError::ReqHandlerError(io::Error::new(io::ErrorKind::Other, e))
+            })?;
         self.memory = Some(Memory { mappings });
 
         Ok(())
@@ -712,13 +623,11 @@ impl<S: VhostUserBackend> VhostUserSlaveReqHandler for VhostUserHandler<S> {
         // VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
         // VHOST_USER_GET_VRING_BASE.
         self.vrings[index as usize].write().unwrap().queue.ready = false;
-        self.vring_handler
-            .read()
-            .unwrap()
-            .unregister_vring_listener(index as usize)
-            .map_err(|e| {
-                VhostUserError::ReqHandlerError(io::Error::new(io::ErrorKind::Other, e))
-            })?;
+        if let Some(fd) = self.vrings[index as usize].read().unwrap().kick.as_ref() {
+            self.worker
+                .unregister_listener(fd.as_raw_fd(), epoll::Events::EPOLLIN, index as u64)
+                .map_err(VhostUserError::ReqHandlerError)?;
+        }
 
         let next_avail = self.vrings[index as usize]
             .read()
@@ -748,13 +657,11 @@ impl<S: VhostUserBackend> VhostUserSlaveReqHandler for VhostUserHandler<S> {
         // VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
         // VHOST_USER_GET_VRING_BASE.
         self.vrings[index as usize].write().unwrap().queue.ready = true;
-        self.vring_handler
-            .read()
-            .unwrap()
-            .register_vring_listener(index as usize)
-            .map_err(|e| {
-                VhostUserError::ReqHandlerError(io::Error::new(io::ErrorKind::Other, e))
-            })?;
+        if let Some(fd) = self.vrings[index as usize].read().unwrap().kick.as_ref() {
+            self.worker
+                .register_listener(fd.as_raw_fd(), epoll::Events::EPOLLIN, index as u64)
+                .map_err(VhostUserError::ReqHandlerError)?;
+        }
 
         Ok(())
     }