QMP: Introduce Human Monitor passthrough command

This command allows QMP clients to execute HMP commands.

Please, check the documentation added to the qmp-commands.hx file
for additional details about the interface and its limitations.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -491,6 +491,44 @@ static int do_qmp_capabilities(Monitor *mon, const QDict *params,
     return 0;
 }
 
+static int mon_set_cpu(int cpu_index);
+static void handle_user_command(Monitor *mon, const char *cmdline);
+
+static int do_hmp_passthrough(Monitor *mon, const QDict *params,
+                              QObject **ret_data)
+{
+    int ret = 0;
+    Monitor *old_mon, hmp;
+    CharDriverState mchar;
+
+    memset(&hmp, 0, sizeof(hmp));
+    qemu_chr_init_mem(&mchar);
+    hmp.chr = &mchar;
+
+    old_mon = cur_mon;
+    cur_mon = &hmp;
+
+    if (qdict_haskey(params, "cpu-index")) {
+        ret = mon_set_cpu(qdict_get_int(params, "cpu-index"));
+        if (ret < 0) {
+            cur_mon = old_mon;
+            qerror_report(QERR_INVALID_PARAMETER_VALUE, "cpu-index", "a CPU number");
+            goto out;
+        }
+    }
+
+    handle_user_command(&hmp, qdict_get_str(params, "command-line"));
+    cur_mon = old_mon;
+
+    if (qemu_chr_mem_osize(hmp.chr) > 0) {
+        *ret_data = QOBJECT(qemu_chr_mem_to_qs(hmp.chr));
+    }
+
+out:
+    qemu_chr_close_mem(hmp.chr);
+    return ret;
+}
+
 static int compare_cmd(const char *name, const char *list)
 {
     const char *p, *pstart;

qmp-commands.hx

@@ -761,6 +761,51 @@ Example:
 
 Note: This command must be issued before issuing any other command.
 
+EQMP
+
+    {
+        .name       = "human-monitor-command",
+        .args_type  = "command-line:s,cpu-index:i?",
+        .params     = "",
+        .help       = "",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_hmp_passthrough,
+    },
+
+SQMP
+human-monitor-command
+---------------------
+
+Execute a Human Monitor command.
+
+Arguments: 
+
+- command-line: the command name and its arguments, just like the
+                Human Monitor's shell (json-string)
+- cpu-index: select the CPU number to be used by commands which access CPU
+             data, like 'info registers'. The Monitor selects CPU 0 if this
+             argument is not provided (json-int, optional)
+
+Example:
+
+-> { "execute": "human-monitor-command", "arguments": { "command-line": "info kvm" } }
+<- { "return": "kvm support: enabled\r\n" }
+
+Notes:
+
+(1) The Human Monitor is NOT an stable interface, this means that command
+    names, arguments and responses can change or be removed at ANY time.
+    Applications that rely on long term stability guarantees should NOT
+    use this command
+
+(2) Limitations:
+
+    o This command is stateless, this means that commands that depend
+      on state information (such as getfd) might not work
+
+    o Commands that prompt the user for data (eg. 'cont' when the block
+      device is encrypted) don't currently work
+
 3. Query Commands
 =================