misc: Make Bus::write() return an Option<Arc<Barrier>>

This can be uses to indicate to the caller that it should wait on the
barrier before returning as there is some asynchronous activity
triggered by the write which requires the KVM exit to block until it's
completed.

This is useful for having vCPU thread wait for the VMM thread to proceed
to activate the virtio devices.

See #1863

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-device/src/bus.rs

@@ -9,7 +9,7 @@
 
 use std::cmp::{Ord, Ordering, PartialEq, PartialOrd};
 use std::collections::btree_map::BTreeMap;
-use std::sync::{Arc, Mutex, RwLock, Weak};
+use std::sync::{Arc, Barrier, Mutex, RwLock, Weak};
 use std::{convert, error, fmt, io, result};
 
 /// Trait for devices that respond to reads or writes in an arbitrary address space.
@@ -21,7 +21,9 @@ pub trait BusDevice: Send {
     /// Reads at `offset` from this device
     fn read(&mut self, base: u64, offset: u64, data: &mut [u8]) {}
     /// Writes at `offset` into this device
-    fn write(&mut self, base: u64, offset: u64, data: &[u8]) {}
+    fn write(&mut self, base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
+        None
+    }
     /// Triggers the `irq_mask` interrupt on this device
     fn interrupt(&self, irq_mask: u32) {}
 }
@@ -257,10 +259,12 @@ mod tests {
             }
         }
 
-        fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+        fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
             for (i, v) in data.iter().enumerate() {
                 assert_eq!(*v, (offset as u8) + (i as u8))
             }
+
+            None
         }
     }