vm-virtio: Implement the Pausable trait for all virtio devices

Due to the amount of code currently duplicated across virtio devices,
the stats for this commit is on the large side but it's mostly more
duplicated code, unfortunately.

Migratable and Snapshotable placeholder implementations are provided as
well, making all virtio devices Migratable.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-virtio/src/rng.rs

@@ -9,6 +9,7 @@ use std::fs::File;
 use std::io;
 use std::os::unix::io::AsRawFd;
 use std::result;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, RwLock};
 use std::thread;
 
@@ -18,6 +19,7 @@ use super::{
     VIRTIO_F_IOMMU_PLATFORM, VIRTIO_F_VERSION_1,
 };
 use crate::{VirtioInterrupt, VirtioInterruptType};
+use vm_device::{Migratable, MigratableError, Pausable, Snapshotable};
 use vm_memory::{Bytes, GuestMemoryMmap};
 use vmm_sys_util::eventfd::EventFd;
 
@@ -29,6 +31,8 @@ const QUEUE_SIZES: &[u16] = &[QUEUE_SIZE];
 const QUEUE_AVAIL_EVENT: DeviceEventT = 0;
 // The device has been dropped.
 const KILL_EVENT: DeviceEventT = 1;
+// The device should be paused.
+const PAUSE_EVENT: DeviceEventT = 2;
 
 struct RngEpollHandler {
     queues: Vec<Queue>,
@@ -37,6 +41,7 @@ struct RngEpollHandler {
     interrupt_cb: Arc<VirtioInterrupt>,
     queue_evt: EventFd,
     kill_evt: EventFd,
+    pause_evt: EventFd,
 }
 
 impl RngEpollHandler {
@@ -81,7 +86,7 @@ impl RngEpollHandler {
         })
     }
 
-    fn run(&mut self) -> result::Result<(), DeviceError> {
+    fn run(&mut self, paused: Arc<AtomicBool>) -> result::Result<(), DeviceError> {
         // Create the epoll file descriptor
         let epoll_fd = epoll::create(true).map_err(DeviceError::EpollCreateFd)?;
 
@@ -100,6 +105,13 @@ impl RngEpollHandler {
             epoll::Event::new(epoll::Events::EPOLLIN, u64::from(KILL_EVENT)),
         )
         .map_err(DeviceError::EpollCtl)?;
+        epoll::ctl(
+            epoll_fd,
+            epoll::ControlOptions::EPOLL_CTL_ADD,
+            self.pause_evt.as_raw_fd(),
+            epoll::Event::new(epoll::Events::EPOLLIN, u64::from(PAUSE_EVENT)),
+        )
+        .map_err(DeviceError::EpollCtl)?;
 
         const EPOLL_EVENTS_LEN: usize = 100;
         let mut events = vec![epoll::Event::new(epoll::Events::empty(), 0); EPOLL_EVENTS_LEN];
@@ -141,6 +153,15 @@ impl RngEpollHandler {
                         debug!("KILL_EVENT received, stopping epoll loop");
                         break 'epoll;
                     }
+                    PAUSE_EVENT => {
+                        debug!("PAUSE_EVENT received, pausing virtio-rng epoll loop");
+                        // We loop here to handle spurious park() returns.
+                        // Until we have not resumed, the paused boolean will
+                        // be true.
+                        while paused.load(Ordering::SeqCst) {
+                            thread::park();
+                        }
+                    }
                     _ => {
                         error!("Unknown event for virtio-block");
                     }
@@ -155,11 +176,14 @@ impl RngEpollHandler {
 /// Virtio device for exposing entropy to the guest OS through virtio.
 pub struct Rng {
     kill_evt: Option<EventFd>,
+    pause_evt: Option<EventFd>,
     random_file: Option<File>,
     avail_features: u64,
     acked_features: u64,
     queue_evts: Option<Vec<EventFd>>,
     interrupt_cb: Option<Arc<VirtioInterrupt>>,
+    epoll_thread: Option<thread::JoinHandle<result::Result<(), DeviceError>>>,
+    paused: Arc<AtomicBool>,
 }
 
 impl Rng {
@@ -174,11 +198,14 @@ impl Rng {
 
         Ok(Rng {
             kill_evt: None,
+            pause_evt: None,
             random_file: Some(random_file),
             avail_features,
             acked_features: 0u64,
             queue_evts: None,
             interrupt_cb: None,
+            epoll_thread: None,
+            paused: Arc::new(AtomicBool::new(false)),
         })
     }
 }
@@ -259,16 +286,22 @@ impl VirtioDevice for Rng {
             return Err(ActivateError::BadActivate);
         }
 
-        let (self_kill_evt, kill_evt) =
-            match EventFd::new(EFD_NONBLOCK).and_then(|e| Ok((e.try_clone()?, e))) {
-                Ok(v) => v,
-                Err(e) => {
-                    error!("failed creating kill EventFd pair: {}", e);
-                    return Err(ActivateError::BadActivate);
-                }
-            };
+        let (self_kill_evt, kill_evt) = EventFd::new(EFD_NONBLOCK)
+            .and_then(|e| Ok((e.try_clone()?, e)))
+            .map_err(|e| {
+                error!("failed creating kill EventFd pair: {}", e);
+                ActivateError::BadActivate
+            })?;
         self.kill_evt = Some(self_kill_evt);
 
+        let (self_pause_evt, pause_evt) = EventFd::new(EFD_NONBLOCK)
+            .and_then(|e| Ok((e.try_clone()?, e)))
+            .map_err(|e| {
+                error!("failed creating pause EventFd pair: {}", e);
+                ActivateError::BadActivate
+            })?;
+        self.pause_evt = Some(self_pause_evt);
+
         // Save the interrupt EventFD as we need to return it on reset
         // but clone it to pass into the thread.
         self.interrupt_cb = Some(interrupt_cb.clone());
@@ -296,16 +329,18 @@ impl VirtioDevice for Rng {
                 interrupt_cb,
                 queue_evt: queue_evts.remove(0),
                 kill_evt,
+                pause_evt,
             };
 
-            let worker_result = thread::Builder::new()
+            let paused = self.paused.clone();
+            thread::Builder::new()
                 .name("virtio_rng".to_string())
-                .spawn(move || handler.run());
-
-            if let Err(e) = worker_result {
-                error!("failed to spawn virtio_rng worker: {}", e);
-                return Err(ActivateError::BadActivate);
-            }
+                .spawn(move || handler.run(paused))
+                .map(|thread| self.epoll_thread = Some(thread))
+                .map_err(|e| {
+                    error!("failed to clone the virtio-rng epoll thread: {}", e);
+                    ActivateError::BadActivate
+                })?;
 
             return Ok(());
         }
@@ -313,6 +348,12 @@ impl VirtioDevice for Rng {
     }
 
     fn reset(&mut self) -> Option<(Arc<VirtioInterrupt>, Vec<EventFd>)> {
+        // We first must resume the virtio thread if it was paused.
+        if self.pause_evt.take().is_some() {
+            self.resume().ok()?;
+        }
+
+        // Then kill it.
         if let Some(kill_evt) = self.kill_evt.take() {
             // Ignore the result because there is nothing we can do about it.
             let _ = kill_evt.write(1);
@@ -325,3 +366,7 @@ impl VirtioDevice for Rng {
         ))
     }
 }
+
+virtio_pausable!(Rng);
+impl Snapshotable for Rng {}
+impl Migratable for Rng {}