virtio: console: add flow control

The virtio-serial-bus already has the logic to make flow control work
properly.  Hook into the char layer's new ability to signal a backend is
writable again.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Message-id: abffa02235d55ca6e2489068c58971c8897e976c.1362505276.git.amit.shah@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-console.c

@@ -20,6 +20,18 @@ typedef struct VirtConsole {
     CharDriverState *chr;
 } VirtConsole;
 
+/*
+ * Callback function that's called from chardevs when backend becomes
+ * writable.
+ */
+static gboolean chr_write_unblocked(GIOChannel *chan, GIOCondition cond,
+                                    void *opaque)
+{
+    VirtConsole *vcon = opaque;
+
+    virtio_serial_throttle_port(&vcon->port, false);
+    return FALSE;
+}
 
 /* Callback function that's called when the guest sends us data */
 static ssize_t flush_buf(VirtIOSerialPort *port, const uint8_t *buf, size_t len)
@@ -48,6 +60,7 @@ static ssize_t flush_buf(VirtIOSerialPort *port, const uint8_t *buf, size_t len)
          * do_flush_queued_data().
          */
         ret = 0;
+        qemu_chr_fe_add_watch(vcon->chr, G_IO_OUT, chr_write_unblocked, vcon);
     }
     return ret;
 }