contrib/plugins: add plugin showcasing new dicontinuity related API

We recently introduced new plugin API for registration of discontinuity
related callbacks. This change introduces a minimal plugin showcasing
the new API. It simply counts the occurances of interrupts, exceptions
and host calls per CPU and reports the counts when exitting.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Julian Ganz <neither@nut.email>
Message-ID: <20251027110344.2289945-11-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

contrib/plugins/meson.build

@@ -1,6 +1,6 @@
 contrib_plugins = ['bbv', 'cache', 'cflow', 'drcov', 'execlog', 'hotblocks',
                    'hotpages', 'howvec', 'hwprofile', 'ips', 'stoptrigger',
-                   'uftrace']
+                   'traps', 'uftrace']
 if host_os != 'windows'
   # lockstep uses socket.h
   contrib_plugins += 'lockstep'

contrib/plugins/traps.c

@@ -0,0 +1,83 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright (C) 2025, Julian Ganz <neither@nut.email>
+ *
+ * Traps - count traps
+ *
+ * Count the number of interrupts (asyncronous events), exceptions (synchronous
+ * events) and host calls (e.g. semihosting) per cpu and report those counts on
+ * exit.
+ */
+
+#include <stdio.h>
+
+#include <qemu-plugin.h>
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
+
+typedef struct {
+    uint64_t interrupts;
+    uint64_t exceptions;
+    uint64_t hostcalls;
+} TrapCounters;
+
+static struct qemu_plugin_scoreboard *traps;
+
+static void vcpu_discon(qemu_plugin_id_t id, unsigned int vcpu_index,
+                        enum qemu_plugin_discon_type type, uint64_t from_pc,
+                        uint64_t to_pc)
+{
+    TrapCounters *rec = qemu_plugin_scoreboard_find(traps, vcpu_index);
+    switch (type) {
+    case QEMU_PLUGIN_DISCON_INTERRUPT:
+        rec->interrupts++;
+        break;
+    case QEMU_PLUGIN_DISCON_EXCEPTION:
+        rec->exceptions++;
+        break;
+    case QEMU_PLUGIN_DISCON_HOSTCALL:
+        rec->hostcalls++;
+        break;
+    default:
+        g_assert_not_reached();
+        break;
+    }
+}
+
+static void plugin_exit(qemu_plugin_id_t id, void *p)
+{
+    g_autoptr(GString) report;
+    report = g_string_new("VCPU, interrupts, exceptions, hostcalls\n");
+    int max_vcpus = qemu_plugin_num_vcpus();
+    int vcpu;
+
+    for (vcpu = 0; vcpu < max_vcpus; vcpu++) {
+        TrapCounters *rec = qemu_plugin_scoreboard_find(traps, vcpu);
+        g_string_append_printf(report,
+                               "% 4d, % 10"PRId64", % 10"PRId64", % 10"PRId64
+                               "\n", vcpu, rec->interrupts, rec->exceptions,
+                               rec->hostcalls);
+    }
+
+    qemu_plugin_outs(report->str);
+    qemu_plugin_scoreboard_free(traps);
+}
+
+QEMU_PLUGIN_EXPORT
+int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
+                        int argc, char **argv)
+{
+    if (!info->system_emulation) {
+        qemu_plugin_outs("Note: interrupts are only reported in system"
+                         " emulation mode.");
+    }
+
+    traps = qemu_plugin_scoreboard_new(sizeof(TrapCounters));
+
+    qemu_plugin_register_vcpu_discon_cb(id, QEMU_PLUGIN_DISCON_ALL,
+                                        vcpu_discon);
+
+    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
+
+    return 0;
+}

docs/about/emulation.rst

@@ -1015,6 +1015,14 @@ interesting to generate them around a particular point of execution::
     # generate trace around init execution (2 seconds):
     $ uftrace dump --chrome --time-range=1753122320~1753122322 > init.json
 
+Count traps
+...........
+
+``contrib/plugins/traps.c``
+
+This plugin counts the number of interrupts (asyncronous events), exceptions
+(synchronous events) and host calls (e.g. semihosting) per cpu.
+
 Other emulation features
 ------------------------