virtio-devices: vsock: RST vsocks on snapshot restore

Otherwise guest connections just hang.

Signed-off-by: Peter Oskolkov <posk@google.com>

virtio-devices/src/vsock/device.rs

@@ -323,6 +323,8 @@ pub struct Vsock<B: VsockBackend> {
 pub struct VsockState {
     pub avail_features: u64,
     pub acked_features: u64,
+    #[serde(default)]
+    pub connections: Vec<(u32, u32)>,
 }
 
 impl<B> Vsock<B>
@@ -336,7 +338,7 @@ where
         id: String,
         cid: u32,
         path: PathBuf,
-        backend: B,
+        mut backend: B,
         iommu: bool,
         seccomp_action: SeccompAction,
         exit_evt: EventFd,
@@ -344,6 +346,9 @@ where
     ) -> io::Result<Vsock<B>> {
         let (avail_features, acked_features, paused) = if let Some(state) = state {
             info!("Restoring virtio-vsock {id}");
+            // Instead of letting the guest connection hang/timeout, proactively let
+            // the guest know the connection is gone.
+            backend.queue_rst_for_connections(state.connections.clone());
             (state.avail_features, state.acked_features, true)
         } else {
             let mut avail_features = (1u64 << VIRTIO_F_VERSION_1) | (1u64 << VIRTIO_F_IN_ORDER);
@@ -378,6 +383,7 @@ where
         VsockState {
             avail_features: self.common.avail_features,
             acked_features: self.common.acked_features,
+            connections: self.backend.read().unwrap().connections(),
         }
     }
 

virtio-devices/src/vsock/mod.rs

@@ -158,7 +158,12 @@ pub trait VsockChannel {
 /// It that needs to be sendable through a mpsc channel (the latter due to how `vmm::EpollContext` works).
 /// Currently, the only implementation we have is `crate::virtio::unix::muxer::VsockMuxer`, which
 /// translates guest-side vsock connections to host-side Unix domain socket connections.
-pub trait VsockBackend: VsockChannel + VsockEpollListener + Send {}
+pub trait VsockBackend: VsockChannel + VsockEpollListener + Send {
+    fn connections(&self) -> Vec<(u32, u32)> {
+        Vec::new()
+    }
+    fn queue_rst_for_connections(&mut self, _conns: Vec<(u32, u32)>) {}
+}
 
 #[cfg(any(test, fuzzing))]
 pub mod unit_tests {

virtio-devices/src/vsock/unix/muxer.rs

@@ -345,7 +345,23 @@ impl VsockEpollListener for VsockMuxer {
     }
 }
 
-impl VsockBackend for VsockMuxer {}
+impl VsockBackend for VsockMuxer {
+    fn connections(&self) -> Vec<(u32, u32)> {
+        self.conn_map
+            .keys()
+            .map(|k| (k.local_port, k.peer_port))
+            .collect()
+    }
+
+    fn queue_rst_for_connections(&mut self, conns: Vec<(u32, u32)>) {
+        for (local_port, peer_port) in conns {
+            self.rxq.push(MuxerRx::RstPkt {
+                local_port,
+                peer_port,
+            });
+        }
+    }
+}
 
 impl VsockMuxer {
     /// Muxer constructor.