diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
index 8dbc6fcb89..bc42b5939d 100644
--- a/include/qemu/main-loop.h
+++ b/include/qemu/main-loop.h
@@ -242,9 +242,33 @@ 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);
+
 /**
  * qemu_mutex_lock_iothread: Lock the main loop mutex.
  *
diff --git a/softmmu/cpus.c b/softmmu/cpus.c
index d1ff3cfea1..1681844b61 100644
--- a/softmmu/cpus.c
+++ b/softmmu/cpus.c
@@ -481,6 +481,11 @@ bool qemu_mutex_iothread_locked(void)
     return get_iothread_locked();
 }
 
+bool qemu_in_main_thread(void)
+{
+    return qemu_mutex_iothread_locked();
+}
+
 /*
  * The BQL is taken from so many places that it is worth profiling the
  * callers directly, instead of funneling them all through a single function.
diff --git a/stubs/iothread-lock-block.c b/stubs/iothread-lock-block.c
new file mode 100644
index 0000000000..c88ed70462
--- /dev/null
+++ b/stubs/iothread-lock-block.c
@@ -0,0 +1,8 @@
+#include "qemu/osdep.h"
+#include "qemu/main-loop.h"
+
+bool qemu_in_main_thread(void)
+{
+    return qemu_get_current_aio_context() == qemu_get_aio_context();
+}
+
diff --git a/stubs/meson.build b/stubs/meson.build
index d359cbe1ad..6f80fec761 100644
--- a/stubs/meson.build
+++ b/stubs/meson.build
@@ -17,6 +17,9 @@ if linux_io_uring.found()
   stub_ss.add(files('io_uring.c'))
 endif
 stub_ss.add(files('iothread-lock.c'))
+if have_block
+  stub_ss.add(files('iothread-lock-block.c'))
+endif
 stub_ss.add(files('isa-bus.c'))
 stub_ss.add(files('is-daemonized.c'))
 if libaio.found()