When gdb_set_stop_cpu() is called with a CPU associated to a process
currently not attached by the GDB client, return without modifying the
stop CPU. Otherwise, GDB get confused if it receives packets with a
thread-id it does not know about.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
---
 gdbstub.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gdbstub.c b/gdbstub.c
index 36ed7081ea..6a55bf2785 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1759,10 +1759,19 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
     return RS_IDLE;
 }
 
 void gdb_set_stop_cpu(CPUState *cpu)
 {
+    GDBProcess *p = gdb_get_cpu_process(gdbserver_state, cpu);
+
+    if (!p->attached) {
+        /* Having a stop CPU corresponding to a process that is not attached
+         * confuses GDB. So we ignore the request.
+         */
+        return;
+    }
+
     gdbserver_state->c_cpu = cpu;
     gdbserver_state->g_cpu = cpu;
 }
 
 #ifndef CONFIG_USER_ONLY
-- 
2.18.0