sound: add latency_bytes field to IOMessage

The latency field specifies how long it will take until the device
finishes playing the I/O message's buffers.

Since the I/O message is dropped as soon as the last bytes are copied
over to the host's internal buffers, assume the message is dropped
almost after the host has started playing this buffer.

This solves the issue of a guest app exiting before the host has done
playing audio because it kept sending new buffers as fast as possible
without waiting (latency).

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

staging/vhost-device-sound/src/device.rs

@@ -483,6 +483,7 @@ impl VhostUserSoundThread {
             let message = Arc::new(IOMessage {
                 vring: vring.clone(),
                 status: VIRTIO_SND_S_OK.into(),
+                latency_bytes: 0.into(),
                 desc_chain: desc_chain.clone(),
                 descriptor: descriptors.last().cloned().unwrap(),
             });

staging/vhost-device-sound/src/lib.rs

@@ -248,6 +248,7 @@ impl SoundConfig {
 
 pub struct IOMessage {
     status: std::sync::atomic::AtomicU32,
+    pub latency_bytes: std::sync::atomic::AtomicU32,
     desc_chain: SoundDescriptorChain,
     descriptor: virtio_queue::Descriptor,
     vring: VringRwLock,
@@ -258,7 +259,10 @@ impl Drop for IOMessage {
         log::trace!("dropping IOMessage");
         let resp = VirtioSoundPcmStatus {
             status: self.status.load(std::sync::atomic::Ordering::SeqCst).into(),
-            latency_bytes: 0.into(),
+            latency_bytes: self
+                .latency_bytes
+                .load(std::sync::atomic::Ordering::SeqCst)
+                .into(),
         };
 
         if let Err(err) = self

staging/vhost-device-sound/src/stream.rs

@@ -279,6 +279,10 @@ impl Buffer {
 
 impl Drop for Buffer {
     fn drop(&mut self) {
+        self.message.latency_bytes.fetch_add(
+            self.data_descriptor.len(),
+            std::sync::atomic::Ordering::SeqCst,
+        );
         log::trace!("dropping buffer {:?}", self);
     }
 }