gdbstub: Clarify what gdb_handlesig() is doing

gdb_handlesig()'s behaviour is not entirely obvious at first
glance. Add a doc comment for it, and also add a comment
explaining why it's ok for gdb_do_syscallv() to ignore
gdb_handlesig()'s return value. (Coverity complains about
this: CID 1390850.)
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-Id: <20180515181958.25837-1-peter.maydell@linaro.org>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

gdbstub.c

@@ -1558,6 +1558,12 @@ void gdb_do_syscallv(gdb_syscall_complete_cb cb, const char *fmt, va_list va)
     *p = 0;
 #ifdef CONFIG_USER_ONLY
     put_packet(s, s->syscall_buf);
+    /* Return control to gdb for it to process the syscall request.
+     * Since the protocol requires that gdb hands control back to us
+     * using a "here are the results" F packet, we don't need to check
+     * gdb_handlesig's return value (which is the signal to deliver if
+     * execution was resumed via a continue packet).
+     */
     gdb_handlesig(s->c_cpu, 0);
 #else
     /* In this case wait to send the syscall packet until notification that

include/exec/gdbstub.h

@@ -48,6 +48,21 @@ int use_gdb_syscalls(void);
 void gdb_set_stop_cpu(CPUState *cpu);
 void gdb_exit(CPUArchState *, int);
 #ifdef CONFIG_USER_ONLY
+/**
+ * gdb_handlesig: yield control to gdb
+ * @cpu: CPU
+ * @sig: if non-zero, the signal number which caused us to stop
+ *
+ * This function yields control to gdb, when a user-mode-only target
+ * needs to stop execution. If @sig is non-zero, then we will send a
+ * stop packet to tell gdb that we have stopped because of this signal.
+ *
+ * This function will block (handling protocol requests from gdb)
+ * until gdb tells us to continue target execution. When it does
+ * return, the return value is a signal to deliver to the target,
+ * or 0 if no signal should be delivered, ie the signal that caused
+ * us to stop should be ignored.
+ */
 int gdb_handlesig(CPUState *, int);
 void gdb_signalled(CPUArchState *, int);
 void gdbserver_fork(CPUState *);