plugins: new syscalls plugin

This commit adds a new syscalls plugin that displays the syscalls
as they are executed and returned. This plugin outputs the number
of the syscall as well as the syscall return value.

Works in *-user only.

Essentially, this commit restores:

  https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg00846.html

by using the new QEMU plugin API.

Signed-off-by: Matthias Weckbecker <matthias@weckbecker.name>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200812115816.4454-1-matthias@weckbecker.name>
Message-Id: <20210312172821.31647-2-alex.bennee@linaro.org>

tests/plugin/meson.build

@@ -1,5 +1,5 @@
 t = []
-foreach i : ['bb', 'empty', 'insn', 'mem']
+foreach i : ['bb', 'empty', 'insn', 'mem', 'syscall']
   t += shared_module(i, files(i + '.c'),
                      include_directories: '../../include/qemu',
                      dependencies: glib)

tests/plugin/syscall.c

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2020, Matthias Weckbecker <matthias@weckbecker.name>
+ *
+ * License: GNU GPL, version 2 or later.
+ *   See the COPYING file in the top-level directory.
+ */
+#include <inttypes.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <glib.h>
+
+#include <qemu-plugin.h>
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
+
+static void vcpu_syscall(qemu_plugin_id_t id, unsigned int vcpu_index,
+                         int64_t num, uint64_t a1, uint64_t a2,
+                         uint64_t a3, uint64_t a4, uint64_t a5,
+                         uint64_t a6, uint64_t a7, uint64_t a8)
+{
+    g_autofree gchar *out = g_strdup_printf("syscall #%" PRIi64 "\n", num);
+    qemu_plugin_outs(out);
+}
+
+static void vcpu_syscall_ret(qemu_plugin_id_t id, unsigned int vcpu_idx,
+                             int64_t num, int64_t ret)
+{
+    g_autofree gchar *out;
+    out = g_strdup_printf("syscall #%" PRIi64 " returned -> %" PRIi64 "\n",
+            num, ret);
+    qemu_plugin_outs(out);
+}
+
+/* ************************************************************************* */
+
+static void plugin_exit(qemu_plugin_id_t id, void *p) {}
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
+                                           const qemu_info_t *info,
+                                           int argc, char **argv)
+{
+    qemu_plugin_register_vcpu_syscall_cb(id, vcpu_syscall);
+    qemu_plugin_register_vcpu_syscall_ret_cb(id, vcpu_syscall_ret);
+    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
+    return 0;
+}