target-arm: Fix gdb singlestep handling in arm_debug_excp_handler()

Do not raise a CPU exception if no CPU breakpoint has fired, since
singlestep is also done by generating a debug internal exception. This
fixes a bug with singlestepping in gdbstub.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1446726361-18328-1-git-send-email-serge.fdrv@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target-arm/op_helper.c

@@ -917,7 +917,13 @@ void arm_debug_excp_handler(CPUState *cs)
         uint64_t pc = is_a64(env) ? env->pc : env->regs[15];
         bool same_el = (arm_debug_target_el(env) == arm_current_el(env));
 
-        if (cpu_breakpoint_test(cs, pc, BP_GDB)) {
+        /* (1) GDB breakpoints should be handled first.
+         * (2) Do not raise a CPU exception if no CPU breakpoint has fired,
+         * since singlestep is also done by generating a debug internal
+         * exception.
+         */
+        if (cpu_breakpoint_test(cs, pc, BP_GDB)
+            || !cpu_breakpoint_test(cs, pc, BP_CPU)) {
             return;
         }