virtio-serial-bus: bump up control vq size to 32

The current default of 16 buffers for the control vq is too small.  We
can get more entries in there, for example when asking the guest to add
max. allowed ports.

Note: a more robust solution would involve some kind of event queueing
in host to guarantee no event loss. Added a TODO to look into
this later.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio-serial-bus.c

@@ -769,10 +769,16 @@ VirtIODevice *virtio_serial_init(DeviceState *dev, uint32_t max_nr_ports)
     /* Add a queue for guest to host transfers for port 0 (backward compat) */
     vser->ovqs[0] = virtio_add_queue(vdev, 128, handle_output);
 
+    /* TODO: host to guest notifications can get dropped
+     * if the queue fills up. Implement queueing in host,
+     * this might also make it possible to reduce the control
+     * queue size: as guest preposts buffers there,
+     * this will save 4Kbyte of guest memory per entry. */
+
     /* control queue: host to guest */
-    vser->c_ivq = virtio_add_queue(vdev, 16, control_in);
+    vser->c_ivq = virtio_add_queue(vdev, 32, control_in);
     /* control queue: guest to host */
-    vser->c_ovq = virtio_add_queue(vdev, 16, control_out);
+    vser->c_ovq = virtio_add_queue(vdev, 32, control_out);
 
     for (i = 1; i < vser->bus->max_nr_ports; i++) {
         /* Add a per-port queue for host to guest transfers */