1 files changed, 42 insertions, 0 deletions
diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index 8dbc6fcb89..7a4d6a0920 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -242,10 +242,52 @@ AioContext *iohandler_get_aio_context(void);
  * must always be taken outside other locks.  This function helps
  * functions take different paths depending on whether the current
  * thread is running within the main loop mutex.
+ *
+ * This function should never be used in the block layer, because
+ * unit tests, block layer tools and qemu-storage-daemon do not
+ * have a BQL.
+ * Please instead refer to qemu_in_main_thread().
  */
 bool qemu_mutex_iothread_locked(void);
 
 /**
+ * qemu_in_main_thread: return whether it's possible to safely access
+ * the global state of the block layer.
+ *
+ * Global state of the block layer is not accessible from I/O threads
+ * or worker threads; only from threads that "own" the default
+ * AioContext that qemu_get_aio_context() returns.  For tests, block
+ * layer tools and qemu-storage-daemon there is a designated thread that
+ * runs the event loop for qemu_get_aio_context(), and that is the
+ * main thread.
+ *
+ * For emulators, however, any thread that holds the BQL can act
+ * as the block layer main thread; this will be any of the actual
+ * main thread, the vCPU threads or the RCU thread.
+ *
+ * For clarity, do not use this function outside the block layer.
+ */
+bool qemu_in_main_thread(void);
+
+/* Mark and check that the function is part of the global state API. */
+#define GLOBAL_STATE_CODE()                                         \
+    do {                                                            \
+        assert(qemu_in_main_thread());                              \
+    } while (0)
+
+/* Mark and check that the function is part of the I/O API. */
+#define IO_CODE()                                                   \
+    do {                                                            \
+        /* nop */                                                   \
+    } while (0)
+
+/* Mark and check that the function is part of the "I/O OR GS" API. */
+#define IO_OR_GS_CODE()                                             \
+    do {                                                            \
+        /* nop */                                                   \
+    } while (0)
+
+/**
  * qemu_mutex_lock_iothread: Lock the main loop mutex.
  *
  * This function locks the main loop mutex.  The mutex is taken by