diff --git a/tools/ioemu/block-raw.c b/tools/ioemu/block-raw.c
index 41c8abb413..9b01211bbf 100644
--- a/tools/ioemu/block-raw.c
+++ b/tools/ioemu/block-raw.c
@@ -166,7 +166,7 @@ typedef struct RawAIOCB {
     struct RawAIOCB *next;
 } RawAIOCB;
 
-static int aio_sig_num = SIGUSR2;
+const int aio_sig_num = SIGUSR2;
 static RawAIOCB *first_aio; /* AIO issued */
 static int aio_initialized = 0;
 
diff --git a/tools/ioemu/vl.c b/tools/ioemu/vl.c
index 696791f13c..02c3bb21a2 100644
--- a/tools/ioemu/vl.c
+++ b/tools/ioemu/vl.c
@@ -7059,6 +7059,18 @@ int main(int argc, char **argv)
 #endif
 
     char qemu_dm_logfilename[128];
+    
+    /* Ensure that SIGUSR2 is blocked by default when a new thread is created,
+       then only the threads that use the signal unblock it -- this fixes a
+       race condition in Qcow support where the AIO signal is misdelivered.  */
+    {
+        extern const int aio_sig_num;
+        sigset_t set;
+
+        sigemptyset(&set);
+        sigaddset(&set, aio_sig_num);
+        sigprocmask(SIG_BLOCK, &set, NULL);
+    }
 
     LIST_INIT (&vm_change_state_head);
 #ifndef _WIN32