[PATCHv2 1/1] Support monitor chardev hotswap with QMP

[Li Zhang]

For some scenarios, it needs to hot-add a monitor device.
But QEMU doesn't support hotplug yet. It also works by adding
a monitor with null backend by default and then change its
backend to socket by QMP command "chardev-change".

So this patch is to support monitor chardev hotswap with QMP.

Signed-off-by: Li Zhang <li.zh...@cloud.ionos.com>
Reviewed-by: Pankaj Gupta <pankaj.gu...@ionos.com>
---
 v1 -> v2: 
  - Change mutex lock mon_lock section
  - Fix indentation problems

 monitor/monitor-internal.h |  3 +++
 monitor/monitor.c          |  2 +-
 monitor/qmp.c              | 43 +++++++++++++++++++++++++++++++++++---
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 9c3a09cb01..1b80c74883 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -183,4 +183,7 @@ void help_cmd(Monitor *mon, const char *name);
 void handle_hmp_command(MonitorHMP *mon, const char *cmdline);
 int hmp_compare_cmd(const char *name, const char *list);
 
+gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
+                           void *opaque);
+
 #endif
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 636bcc81c5..16a3620d02 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -157,7 +157,7 @@ static inline bool monitor_is_hmp_non_interactive(const 
Monitor *mon)
 
 static void monitor_flush_locked(Monitor *mon);
 
-static gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
+gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
                                   void *opaque)
 {
     Monitor *mon = opaque;
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 2b0308f933..5fa65401ae 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -44,6 +44,7 @@ struct QMPRequest {
     Error *err;
 };
 typedef struct QMPRequest QMPRequest;
+static void monitor_qmp_set_handlers_bh(void *opaque);
 
 QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
 
@@ -477,7 +478,36 @@ void monitor_data_destroy_qmp(MonitorQMP *mon)
     g_queue_free(mon->qmp_requests);
 }
 
-static void monitor_qmp_setup_handlers_bh(void *opaque)
+static int monitor_qmp_change(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+
+    mon->common.use_io_thread = qemu_chr_has_feature(mon->common.chr.chr,
+                                QEMU_CHAR_FEATURE_GCONTEXT);
+
+    if (mon->common.use_io_thread) {
+        aio_bh_schedule_oneshot(iothread_get_aio_context(mon_iothread),
+                                monitor_qmp_set_handlers_bh, mon);
+    } else {
+        qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
+                                 monitor_qmp_read, monitor_qmp_event,
+                                 monitor_qmp_change, &mon->common, NULL, true);
+    }
+
+    qemu_mutex_lock(&mon->common.mon_lock);
+    if (mon->common.out_watch) {
+        g_source_remove(mon->common.out_watch);
+        mon->common.out_watch = qemu_chr_fe_add_watch(&mon->common.chr,
+                                G_IO_OUT | G_IO_HUP,
+                                monitor_unblocked,
+                                &mon->common);
+    }
+    qemu_mutex_unlock(&mon->common.mon_lock);
+
+    return 0;
+}
+
+static void monitor_qmp_set_handlers_bh(void *opaque)
 {
     MonitorQMP *mon = opaque;
     GMainContext *context;
@@ -487,7 +517,14 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
     assert(context);
     qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
                              monitor_qmp_read, monitor_qmp_event,
-                             NULL, &mon->common, context, true);
+                             monitor_qmp_change, &mon->common, context, true);
+
+}
+
+static void monitor_qmp_setup_handlers_bh(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+    monitor_qmp_set_handlers_bh(mon);
     monitor_list_append(&mon->common);
 }
 
@@ -528,7 +565,7 @@ void monitor_init_qmp(Chardev *chr, bool pretty, Error 
**errp)
     } else {
         qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
                                  monitor_qmp_read, monitor_qmp_event,
-                                 NULL, &mon->common, NULL, true);
+                                 monitor_qmp_change, &mon->common, NULL, true);
         monitor_list_append(&mon->common);
     }
 }
-- 
2.25.1