plugins: Add enforcement of QEMU_PLUGIN_CB flags in register R/W callbacks

This patch adds functionality to enforce the requested QEMU_PLUGIN_CB_
flags level passed when registering a callback function using the
plugins API. Each time a callback is about to be invoked, a thread-local
variable will be updated with the level that callback requested. Then,
called API functions (in particular, the register read and write API)
will call qemu_plugin_get_cb_flags() to check the level is at least the
level they require.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Rowan Hart <rowanbhart@gmail.com>
Message-ID: <20250624175351.440780-4-rowanbhart@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250627112512.1880708-8-alex.bennee@linaro.org>

3 files changed, 29 insertions, 6 deletions

diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 33296a1..162a56a 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -368,6 +368,7 @@ typedef struct CPUNegativeOffsetState {
     GArray *plugin_mem_cbs;
     uint64_t plugin_mem_value_low;
     uint64_t plugin_mem_value_high;
+    int32_t plugin_cb_flags;
 #endif
     IcountDecr icount_decr;
     bool can_do_io;
diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
index 9726a9e..f355c7c 100644
--- a/include/qemu/plugin.h
+++ b/include/qemu/plugin.h
@@ -209,6 +209,21 @@ void qemu_plugin_user_prefork_lock(void);
  */
 void qemu_plugin_user_postfork(bool is_child);
 
+enum qemu_plugin_cb_flags tcg_call_to_qemu_plugin_cb_flags(int flags);
+
+static inline void qemu_plugin_set_cb_flags(CPUState *cpu,
+                                            enum qemu_plugin_cb_flags flags)
+{
+    assert(cpu);
+    cpu->neg.plugin_cb_flags = flags;
+}
+
+static inline enum qemu_plugin_cb_flags qemu_plugin_get_cb_flags(void)
+{
+    assert(current_cpu);
+    return current_cpu->neg.plugin_cb_flags;
+}
+
 #else /* !CONFIG_PLUGIN */
 
 static inline void qemu_plugin_add_opts(void)
diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
index cfe1692..9c9ebf6 100644
--- a/include/qemu/qemu-plugin.h
+++ b/include/qemu/qemu-plugin.h
@@ -254,9 +254,6 @@ typedef struct {
  * @QEMU_PLUGIN_CB_NO_REGS: callback does not access the CPU's regs
  * @QEMU_PLUGIN_CB_R_REGS: callback reads the CPU's regs
  * @QEMU_PLUGIN_CB_RW_REGS: callback reads and writes the CPU's regs
- *
- * Note: currently QEMU_PLUGIN_CB_RW_REGS is unused, plugins cannot change
- * system register state.
  */
 enum qemu_plugin_cb_flags {
     QEMU_PLUGIN_CB_NO_REGS,
@@ -901,7 +898,12 @@ GArray *qemu_plugin_get_registers(void);
  * @buf: A GByteArray for the data owned by the plugin
  *
  * This function is only available in a context that register read access is
- * explicitly requested via the QEMU_PLUGIN_CB_R_REGS flag.
+ * explicitly requested via the QEMU_PLUGIN_CB_R_REGS flag, if called inside a
+ * callback that can be registered with a qemu_plugin_cb_flags argument. This
+ * function can also be used in any callback context that does not use a flags
+ * argument, such as in a callback registered with
+ * qemu_plugin_register_vcpu_init_cb(), except for callbacks registered with
+ * qemu_plugin_register_atexit_cb() and qemu_plugin_register_flush_cb().
  *
  * Returns the size of the read register. The content of @buf is in target byte
  * order. On failure returns -1.
@@ -916,8 +918,13 @@ int qemu_plugin_read_register(struct qemu_plugin_register *handle,
  * @handle: a @qemu_plugin_reg_handle handle
  * @buf: A GByteArray for the data owned by the plugin
  *
- * This function is only available in a context that register write access is
- * explicitly requested via the QEMU_PLUGIN_CB_RW_REGS flag.
+ * This function is only available in a context that register read access is
+ * explicitly requested via the QEMU_PLUGIN_CB_RW_REGS flag, if called inside a
+ * callback that can be registered with a qemu_plugin_cb_flags argument. This
+ * function can also be used in any callback context that does not use a flags
+ * argument, such as in a callback registered with
+ * qemu_plugin_register_vcpu_init_cb(), except for callbacks registered with
+ * qemu_plugin_register_atexit_cb() and qemu_plugin_register_flush_cb().
  *
  * The size of @buf must be at least the size of the requested register.
  * Attempting to write a register with @buf smaller than the register size