virtio-devices: Acknowledge a device being paused

Using the Rust Barrier mechanism, this patch forces each virtio device
to acknowledge they've been correctly paused before going further.

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

virtio-devices/src/rng.rs

@@ -18,7 +18,7 @@ use std::io;
 use std::os::unix::io::AsRawFd;
 use std::result;
 use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::Arc;
+use std::sync::{Arc, Barrier};
 use std::thread;
 use vm_memory::{Bytes, GuestAddressSpace, GuestMemoryAtomic, GuestMemoryMmap};
 use vm_migration::{
@@ -88,10 +88,14 @@ impl RngEpollHandler {
             })
     }
 
-    fn run(&mut self, paused: Arc<AtomicBool>) -> result::Result<(), EpollHelperError> {
+    fn run(
+        &mut self,
+        paused: Arc<AtomicBool>,
+        paused_sync: Arc<Barrier>,
+    ) -> result::Result<(), EpollHelperError> {
         let mut helper = EpollHelper::new(&self.kill_evt, &self.pause_evt)?;
         helper.add_event(self.queue_evt.as_raw_fd(), QUEUE_AVAIL_EVENT)?;
-        helper.run(paused, self)?;
+        helper.run(paused, paused_sync, self)?;
 
         Ok(())
     }
@@ -133,6 +137,7 @@ pub struct Rng {
     interrupt_cb: Option<Arc<dyn VirtioInterrupt>>,
     epoll_threads: Option<Vec<thread::JoinHandle<()>>>,
     paused: Arc<AtomicBool>,
+    paused_sync: Arc<Barrier>,
     seccomp_action: SeccompAction,
 }
 
@@ -169,6 +174,7 @@ impl Rng {
             interrupt_cb: None,
             epoll_threads: None,
             paused: Arc::new(AtomicBool::new(false)),
+            paused_sync: Arc::new(Barrier::new(2)),
             seccomp_action,
         })
     }
@@ -288,6 +294,7 @@ impl VirtioDevice for Rng {
             };
 
             let paused = self.paused.clone();
+            let paused_sync = self.paused_sync.clone();
             let mut epoll_threads = Vec::new();
             // Retrieve seccomp filter for virtio_rng thread
             let virtio_rng_seccomp_filter =
@@ -298,7 +305,7 @@ impl VirtioDevice for Rng {
                 .spawn(move || {
                     if let Err(e) = SeccompFilter::apply(virtio_rng_seccomp_filter) {
                         error!("Error applying seccomp filter: {:?}", e);
-                    } else if let Err(e) = handler.run(paused) {
+                    } else if let Err(e) = handler.run(paused, paused_sync) {
                         error!("Error running worker: {:?}", e);
                     }
                 })