virtio-fs: signal exit_evt when guest reports an exit code

When a virtual machine (VM) passes its final exit status via the `KRUN_EXIT_CODE_IOCTL` function on the virtio-fs root filesystem, the host records it in the shared `exit_code` AtomicI32, but does not signal it to the VMM's `EventFd`.

The VMM only stops when `exit_evt` becomes readable, so the VM's reported exit status itself does not wake up the `EventManager` and terminate the VM.

This is a common bug in virtio-fs/VMM exit interactions, not a code path specific to any particular architecture (but the reset path on x86-64 i8042 is configured as a VMM exit event, masking the problem). User-visible symptoms depend on the architecture/platform:

- On platforms without an equivalent rollback mechanism, or where the VM continues to perform reboot/shutdown processing after reporting the exit code, it may continue running even after reporting completion.

We observed the latter scenario on LoongArch, where the virtual machine could continue to restart/shut down, eventually leading to a soft lock rather than the VMM exiting immediately.

Signed-off-by: Zewei Yang <yangzewei@loongson.cn>

Author: yzewei

src/devices/src/virtio/fs/device.rs

@@ -49,6 +49,7 @@ pub struct Fs {
     worker_thread: Option<JoinHandle<()>>,
     worker_stopfd: EventFd,
     exit_code: Arc<AtomicI32>,
+    exit_evt: EventFd,
     #[cfg(target_os = "macos")]
     map_sender: Option<Sender<WorkerMessage>>,
 }
@@ -58,6 +59,7 @@ impl Fs {
         fs_id: String,
         shared_dir: String,
         exit_code: Arc<AtomicI32>,
+        exit_evt: EventFd,
         allow_root_dir_delete: bool,
     ) -> super::Result<Fs> {
         let avail_features = (1u64 << VIRTIO_F_VERSION_1) | (1u64 << VIRTIO_RING_F_EVENT_IDX);
@@ -83,6 +85,7 @@ impl Fs {
             worker_thread: None,
             worker_stopfd: EventFd::new(EFD_NONBLOCK).map_err(FsError::EventFd)?,
             exit_code,
+            exit_evt,
             #[cfg(target_os = "macos")]
             map_sender: None,
         })
@@ -185,6 +188,7 @@ impl VirtioDevice for Fs {
             self.passthrough_cfg.clone(),
             self.worker_stopfd.try_clone().unwrap(),
             self.exit_code.clone(),
+            self.exit_evt.try_clone().unwrap(),
             #[cfg(target_os = "macos")]
             self.map_sender.clone(),
         );

src/devices/src/virtio/fs/worker.rs

@@ -4,7 +4,7 @@ use crossbeam_channel::Sender;
 use utils::worker_message::WorkerMessage;
 
 use std::os::fd::AsRawFd;
-use std::sync::atomic::AtomicI32;
+use std::sync::atomic::{AtomicI32, Ordering};
 use std::sync::Arc;
 use std::thread;
 
@@ -28,6 +28,7 @@ pub struct FsWorker {
     server: Server<PassthroughFs>,
     stop_fd: EventFd,
     exit_code: Arc<AtomicI32>,
+    exit_evt: EventFd,
     #[cfg(target_os = "macos")]
     map_sender: Option<Sender<WorkerMessage>>,
 }
@@ -43,6 +44,7 @@ impl FsWorker {
         passthrough_cfg: passthrough::Config,
         stop_fd: EventFd,
         exit_code: Arc<AtomicI32>,
+        exit_evt: EventFd,
         #[cfg(target_os = "macos")] map_sender: Option<Sender<WorkerMessage>>,
     ) -> Self {
         Self {
@@ -54,6 +56,7 @@ impl FsWorker {
             server: Server::new(PassthroughFs::new(passthrough_cfg).unwrap()),
             stop_fd,
             exit_code,
+            exit_evt,
             #[cfg(target_os = "macos")]
             map_sender,
         }
@@ -155,6 +158,7 @@ impl FsWorker {
                 .map_err(FsError::QueueWriter)
                 .unwrap();
 
+            let exit_evt_before = self.exit_code.load(Ordering::SeqCst);
             if let Err(e) = self.server.handle_message(
                 reader,
                 writer,
@@ -173,6 +177,12 @@ impl FsWorker {
             if queue.needs_notification(&self.mem).unwrap() {
                 self.interrupt.signal_used_queue();
             }
+            let exit_evt_after = self.exit_code.load(Ordering::SeqCst);
+            if exit_evt_before == i32::MAX && exit_evt_after != i32::MAX {
+                if let Err(e) = self.exit_evt.write(exit_evt_after as u64) {
+                    error!("failed to signal exit event: {e:?}");
+                }
+            }
         }
     }
 }

src/vmm/src/builder.rs

@@ -1885,11 +1885,17 @@ fn attach_fs_devices(
     use self::StartMicrovmError::*;
 
     for (i, config) in fs_devs.iter().enumerate() {
+        let exit_evt = vmm
+            .exit_evt
+            .try_clone()
+            .map_err(Error::EventFd)
+            .map_err(StartMicrovmError::Internal)?;
         let fs = Arc::new(Mutex::new(
             devices::virtio::Fs::new(
                 config.fs_id.clone(),
                 config.shared_dir.clone(),
                 exit_code.clone(),
+                exit_evt,
                 config.allow_root_dir_delete,
             )
             .unwrap(),