diff --git a/hw/virtio-console.c b/hw/virtio-console.c
index d0b935491d..62624ec780 100644
--- a/hw/virtio-console.c
+++ b/hw/virtio-console.c
@@ -20,11 +20,11 @@ typedef struct VirtConsole {
 
 
 /* Callback function that's called when the guest sends us data */
-static void flush_buf(VirtIOSerialPort *port, const uint8_t *buf, size_t len)
+static ssize_t flush_buf(VirtIOSerialPort *port, const uint8_t *buf, size_t len)
 {
     VirtConsole *vcon = DO_UPCAST(VirtConsole, port, port);
 
-    qemu_chr_write(vcon->chr, buf, len);
+    return qemu_chr_write(vcon->chr, buf, len);
 }
 
 /* Readiness of the guest to accept data on a port */
diff --git a/hw/virtio-serial.h b/hw/virtio-serial.h
index ff08c40681..9cc0fb3543 100644
--- a/hw/virtio-serial.h
+++ b/hw/virtio-serial.h
@@ -137,10 +137,11 @@ struct VirtIOSerialPortInfo {
 
     /*
      * Guest wrote some data to the port. This data is handed over to
-     * the app via this callback.  The app is supposed to consume all
-     * the data that is presented to it.
+     * the app via this callback.  The app can return a size less than
+     * 'len'.  In this case, throttling will be enabled for this port.
      */
-    void (*have_data)(VirtIOSerialPort *port, const uint8_t *buf, size_t len);
+    ssize_t (*have_data)(VirtIOSerialPort *port, const uint8_t *buf,
+                         size_t len);
 };
 
 /* Interface to the virtio-serial bus */