diff --git a/devices/src/legacy/serial.rs b/devices/src/legacy/serial.rs
index c6554c770..ff4074a27 100644
--- a/devices/src/legacy/serial.rs
+++ b/devices/src/legacy/serial.rs
@@ -124,6 +124,10 @@ impl Serial {
         Self::new(id, interrupt, None)
     }
 
+    pub fn set_out(&mut self, out: Box<dyn io::Write + Send>) {
+        self.out = Some(out);
+    }
+
     /// Queues raw bytes for the guest to read and signals the interrupt if the line status would
     /// change.
     pub fn queue_input_bytes(&mut self, c: &[u8]) -> Result<()> {
@@ -134,6 +138,13 @@ impl Serial {
         Ok(())
     }
 
+    pub fn flush_output(&mut self) -> result::Result<(), io::Error> {
+        if let Some(out) = self.out.as_mut() {
+            out.flush()?;
+        }
+        Ok(())
+    }
+
     fn is_dlab_set(&self) -> bool {
         (self.line_control & LCR_DLAB_BIT) != 0
     }
diff --git a/devices/src/legacy/uart_pl011.rs b/devices/src/legacy/uart_pl011.rs
index e4e2379c7..39e7a1cee 100644
--- a/devices/src/legacy/uart_pl011.rs
+++ b/devices/src/legacy/uart_pl011.rs
@@ -138,6 +138,10 @@ impl Pl011 {
         }
     }
 
+    pub fn set_out(&mut self, out: Box<dyn io::Write + Send>) {
+        self.out = Some(out);
+    }
+
     fn state(&self) -> Pl011State {
         Pl011State {
             flags: self.flags,
@@ -192,6 +196,13 @@ impl Pl011 {
         Ok(())
     }
 
+    pub fn flush_output(&mut self) -> result::Result<(), io::Error> {
+        if let Some(out) = self.out.as_mut() {
+            out.flush()?;
+        }
+        Ok(())
+    }
+
     fn pl011_get_baudrate(&self) -> u32 {
         if self.fbrd == 0 {
             return 0;