plugins: add read-only property for registers

Some registers should be marked as read-only from a plugin API
perspective, as writing to them via qemu_plugin_write_register has no
effect. This includes the program counter, and we expose this fact to
the plugins with this patch.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
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-v7-6-4c3adba52403@epfl.ch
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

include/plugins/qemu-plugin.h

@@ -979,11 +979,14 @@ struct qemu_plugin_register;
  *          writing value with qemu_plugin_write_register
  * @name: register name
  * @feature: optional feature descriptor, can be NULL
+ * @is_readonly: true if the register cannot be written via
+ *               qemu_plugin_write_register
  */
 typedef struct {
     struct qemu_plugin_register *handle;
     const char *name;
     const char *feature;
+    bool is_readonly;
 } qemu_plugin_reg_descriptor;
 
 /**

plugins/api.c

@@ -410,6 +410,12 @@ bool qemu_plugin_bool_parse(const char *name, const char *value, bool *ret)
  * ancillary data the plugin might find useful.
  */
 
+static const char pc_str[] = "pc"; /* generic name for program counter */
+static const char eip_str[] = "eip"; /* x86-specific name for PC */
+static const char rip_str[] = "rip"; /* x86_64-specific name for PC */
+static const char pswa_str[] = "pswa"; /* s390x-specific name for PC */
+static const char iaoq_str[] = "iaoq"; /* HP/PA-specific name for PC */
+static const char rpc_str[] = "rpc"; /* microblaze-specific name for PC */
 static GArray *create_register_handles(GArray *gdbstub_regs)
 {
     GArray *find_data = g_array_new(true, true,
@@ -427,6 +433,16 @@ static GArray *create_register_handles(GArray *gdbstub_regs)
         /* Create a record for the plugin */
         desc.handle = GINT_TO_POINTER(grd->gdb_reg + 1);
         desc.name = g_intern_string(grd->name);
+        desc.is_readonly = false;
+        if (g_strcmp0(desc.name, pc_str) == 0
+            || g_strcmp0(desc.name, eip_str) == 0
+            || g_strcmp0(desc.name, rip_str) == 0
+            || g_strcmp0(desc.name, pswa_str) == 0
+            || g_strcmp0(desc.name, iaoq_str) == 0
+            || g_strcmp0(desc.name, rpc_str) == 0
+           ) {
+            desc.is_readonly = true;
+        }
         desc.feature = g_intern_string(grd->feature_name);
         g_array_append_val(find_data, desc);
     }