plugins: add qemu_plugin_num_vcpus function

We now keep track of how many vcpus were started. This way, a plugin can
easily query number of any vcpus at any point of execution, which
unifies user and system mode workflows.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240213094009.150349-3-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-17-alex.bennee@linaro.org>

include/qemu/qemu-plugin.h

@@ -645,6 +645,9 @@ QEMU_PLUGIN_API
 void qemu_plugin_register_atexit_cb(qemu_plugin_id_t id,
                                     qemu_plugin_udata_cb_t cb, void *userdata);
 
+/* returns how many vcpus were started at this point */
+int qemu_plugin_num_vcpus(void);
+
 /**
  * qemu_plugin_outs() - output string via QEMU's logging system
  * @string: a string

plugins/api.c

@@ -342,6 +342,11 @@ const char *qemu_plugin_hwaddr_device_name(const struct qemu_plugin_hwaddr *h)
 #endif
 }
 
+int qemu_plugin_num_vcpus(void)
+{
+    return plugin_num_vcpus();
+}
+
 /*
  * Plugin output
  */

plugins/core.c

@@ -213,6 +213,7 @@ void qemu_plugin_vcpu_init_hook(CPUState *cpu)
     bool success;
 
     qemu_rec_mutex_lock(&plugin.lock);
+    plugin.num_vcpus = MAX(plugin.num_vcpus, cpu->cpu_index + 1);
     plugin_cpu_update__locked(&cpu->cpu_index, NULL, NULL);
     success = g_hash_table_insert(plugin.cpu_ht, &cpu->cpu_index,
                                   &cpu->cpu_index);
@@ -570,3 +571,8 @@ static void __attribute__((__constructor__)) plugin_init(void)
              QHT_MODE_AUTO_RESIZE);
     atexit(qemu_plugin_atexit_cb);
 }
+
+int plugin_num_vcpus(void)
+{
+    return plugin.num_vcpus;
+}

plugins/plugin.h

@@ -44,6 +44,8 @@ struct qemu_plugin_state {
      * the code cache is flushed.
      */
     struct qht dyn_cb_arr_ht;
+    /* How many vcpus were started */
+    int num_vcpus;
 };
 
 
@@ -97,4 +99,6 @@ void plugin_register_vcpu_mem_cb(GArray **arr,
 
 void exec_inline_op(struct qemu_plugin_dyn_cb *cb);
 
+int plugin_num_vcpus(void);
+
 #endif /* PLUGIN_H */

plugins/qemu-plugins.symbols

@@ -16,6 +16,7 @@
   qemu_plugin_mem_is_sign_extended;
   qemu_plugin_mem_is_store;
   qemu_plugin_mem_size_shift;
+  qemu_plugin_num_vcpus;
   qemu_plugin_outs;
   qemu_plugin_path_to_binary;
   qemu_plugin_register_atexit_cb;