virtio-devices: block: Reduce notification latency when rate limited

When the rate limit was reached it was possible for the notification to
the guest to be lost since the logic to handle the notification was
tightly coupled with processing the queue. The notification would
eventually be triggered when the rate limit pool was refilled but this
could add significant latency.

Address this by refactoring the code to separate processing queue and
signalling - the processing of the queue is suspended when the rate
limit is reached but the signalling will still be attempted if needed
(i.e. VIRTIO_F_EVENT_IDX is still considered.)

Signed-off-by: wuxinyue <wuxinyue.wxy@antgroup.com>
Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

virtio-devices/src/block.rs

@@ -235,11 +235,7 @@ impl BlockEpollHandler {
         Ok(())
     }
 
-    fn process_queue_submit_and_signal(&mut self) -> result::Result<(), EpollHelperError> {
+    fn try_signal_used_queue(&mut self) -> result::Result<(), EpollHelperError> {
-        self.process_queue_submit().map_err(|e| {
-            EpollHelperError::HandleEvent(anyhow!("Failed to process queue (submit): {:?}", e))
-        })?;
-
         if self
             .queue
             .needs_notification(self.mem.memory().deref())
@@ -258,6 +254,14 @@ impl BlockEpollHandler {
         Ok(())
     }
 
+    fn process_queue_submit_and_signal(&mut self) -> result::Result<(), EpollHelperError> {
+        self.process_queue_submit().map_err(|e| {
+            EpollHelperError::HandleEvent(anyhow!("Failed to process queue (submit): {:?}", e))
+        })?;
+
+        self.try_signal_used_queue()
+    }
+
     #[inline]
     fn find_inflight_request(&mut self, completed_head: u16) -> Result<Request> {
         // This loop neatly handles the fast path where the completions are
@@ -514,8 +518,14 @@ impl EpollHelperHandler for BlockEpollHandler {
 
                 // Process the queue only when the rate limit is not reached
                 if !rate_limit_reached {
-                    self.process_queue_submit_and_signal()?
+                    self.process_queue_submit().map_err(|e| {
+                        EpollHelperError::HandleEvent(anyhow!(
+                            "Failed to process queue (submit): {:?}",
+                            e
+                        ))
+                    })?;
                 }
+                self.try_signal_used_queue()?;
             }
             RATE_LIMITER_EVENT => {
                 if let Some(rate_limiter) = &mut self.rate_limiter {