tests/tcg/plugins: test register accesses

The additional plugin tests register accesses, specifically both for
read-only and read-write registers. Writing to a read-only register is
currently not tested, as this would trigger an assertion and fail the
test.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Florian Hofhammer <florian.hofhammer@epfl.ch>
Link: https://lore.kernel.org/qemu-devel/20260305-setpc-v5-v7-8-4c3adba52403@epfl.ch
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

tests/tcg/plugins/meson.build

@@ -6,6 +6,7 @@ test_plugins = [
 'insn.c',
 'mem.c',
 'patch.c',
+'registers.c',
 'reset.c',
 'setpc.c',
 'syscall.c',

tests/tcg/plugins/registers.c

@@ -0,0 +1,79 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ *
+ * Copyright (C) 2026, Florian Hofhammer <florian.hofhammer@epfl.ch>
+ */
+#include "glib.h"
+#include <inttypes.h>
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+
+#include <qemu-plugin.h>
+
+QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
+
+/*
+ * This plugin tests whether we can read and write registers via the plugin
+ * API. We try to just read/write a single register, as some architectures have
+ * registers that cannot be written to, which would fail the test.
+ * See: https://lists.gnu.org/archive/html/qemu-devel/2026-02/msg07025.html
+ */
+static void vcpu_init_cb(qemu_plugin_id_t id, unsigned int vcpu_index)
+{
+    g_autoptr(GArray) regs = qemu_plugin_get_registers();
+    g_assert(regs != NULL);
+    g_autoptr(GByteArray) buf = g_byte_array_sized_new(0);
+    qemu_plugin_reg_descriptor *reg_desc = NULL;
+    bool success = false;
+
+    /* Make sure we can read and write a register not marked as readonly */
+    for (size_t i = 0; i < regs->len; i++) {
+        reg_desc = &g_array_index(regs, qemu_plugin_reg_descriptor, i);
+        if (!reg_desc->is_readonly) {
+            g_byte_array_set_size(buf, 0);
+            success = qemu_plugin_read_register(reg_desc->handle, buf);
+            g_assert(success);
+            g_assert(buf->len > 0);
+            success = qemu_plugin_write_register(reg_desc->handle, buf);
+            g_assert(success);
+            break;
+        } else {
+            reg_desc = NULL;
+        }
+    }
+    g_assert(regs->len == 0 || reg_desc != NULL);
+
+    /*
+     * Check whether we can still read a read-only register. On each
+     * architecture, at least the PC should be read-only because it's only
+     * supposed to be modified via the qemu_plugin_set_pc() function.
+     */
+    for (size_t i = 0; i < regs->len; i++) {
+        reg_desc = &g_array_index(regs, qemu_plugin_reg_descriptor, i);
+        if (reg_desc->is_readonly) {
+            g_byte_array_set_size(buf, 0);
+            success = qemu_plugin_read_register(reg_desc->handle, buf);
+            g_assert(success);
+            g_assert(buf->len > 0);
+            break;
+        } else {
+            reg_desc = NULL;
+        }
+    }
+    g_assert(regs->len == 0 || reg_desc != NULL);
+    /*
+     * Note: we currently do not test whether the read-only register can be
+     * written to, because doing so would throw an assert in the plugin API.
+     */
+}
+
+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_init_cb(id, vcpu_init_cb);
+    return 0;
+}