virtio-serial: report frontend connection state via monitor

Libvirt wants to know about the guest-side connection state of some
virtio-serial ports (in particular the one(s) assigned to guest agent(s)).
Report such states with a new monitor event.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1080376Signed-off-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

hw/char/virtio-console.c

@@ -14,6 +14,7 @@
 #include "qemu/error-report.h"
 #include "trace.h"
 #include "hw/virtio/virtio-serial.h"
+#include "qapi-event.h"
 
 #define TYPE_VIRTIO_CONSOLE_SERIAL_PORT "virtserialport"
 #define VIRTIO_CONSOLE(obj) \
@@ -81,11 +82,16 @@ static ssize_t flush_buf(VirtIOSerialPort *port,
 static void set_guest_connected(VirtIOSerialPort *port, int guest_connected)
 {
     VirtConsole *vcon = VIRTIO_CONSOLE(port);
+    DeviceState *dev = DEVICE(port);
 
-    if (!vcon->chr) {
-        return;
+    if (vcon->chr) {
+        qemu_chr_fe_set_open(vcon->chr, guest_connected);
+    }
+
+    if (dev->id) {
+        qapi_event_send_vserport_change(dev->id, guest_connected,
+                                        &error_abort);
     }
-    qemu_chr_fe_set_open(vcon->chr, guest_connected);
 }
 
 /* Readiness of the guest to accept data on a port */

monitor.c

@@ -588,6 +588,7 @@ static void monitor_qapi_event_init(void)
     monitor_qapi_event_throttle(QAPI_EVENT_BALLOON_CHANGE, 1000);
     monitor_qapi_event_throttle(QAPI_EVENT_QUORUM_REPORT_BAD, 1000);
     monitor_qapi_event_throttle(QAPI_EVENT_QUORUM_FAILURE, 1000);
+    monitor_qapi_event_throttle(QAPI_EVENT_VSERPORT_CHANGE, 1000);
 
     qmp_event_set_func_emit(monitor_qapi_event_queue);
 }

qapi/event.json

@@ -316,3 +316,17 @@
 { 'event': 'QUORUM_REPORT_BAD',
   'data': { '*error': 'str', 'node-name': 'str',
             'sector-num': 'int', 'sector-count': 'int' } }
+
+##
+# @VSERPORT_CHANGE
+#
+# Emitted when the guest opens or closes a virtio-serial port.
+#
+# @id: device identifier of the virtio-serial port
+#
+# @open: true if the guest has opened the virtio-serial port
+#
+# Since: 2.1
+##
+{ 'event': 'VSERPORT_CHANGE',
+  'data': { 'id': 'str', 'open': 'bool' } }