target-arm: Don't halt on WFI unless we don't have any work

Just NOP the WFI instruction if we have work to do.
This doesn't make much difference currently (though it does avoid
jumping out to the top level loop and immediately restarting),
but the distinction between "halt" and "don't halt" will become
more important when the decision to halt requires us to trap
to a higher exception level instead.
Suggested-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEdgar E. Iglesias <edgar.iglesias@xilinx.com>

target-arm/op_helper.c

@@ -252,6 +252,13 @@ void HELPER(wfi)(CPUARMState *env)
 {
     CPUState *cs = CPU(arm_env_get_cpu(env));
 
+    if (cpu_has_work(cs)) {
+        /* Don't bother to go into our "low power state" if
+         * we would just wake up immediately.
+         */
+        return;
+    }
+
     cs->exception_index = EXCP_HLT;
     cs->halted = 1;
     cpu_loop_exit(cs);

target-arm/translate-a64.c

@@ -11113,6 +11113,10 @@ void gen_intermediate_code_internal_a64(ARMCPU *cpu,
              */
             gen_a64_set_pc_im(dc->pc);
             gen_helper_wfi(cpu_env);
+            /* The helper doesn't necessarily throw an exception, but we
+             * must go back to the main loop to check for interrupts anyway.
+             */
+            tcg_gen_exit_tb(0);
             break;
         }
     }

target-arm/translate.c

@@ -11351,6 +11351,10 @@ static inline void gen_intermediate_code_internal(ARMCPU *cpu,
             break;
         case DISAS_WFI:
             gen_helper_wfi(cpu_env);
+            /* The helper doesn't necessarily throw an exception, but we
+             * must go back to the main loop to check for interrupts anyway.
+             */
+            tcg_gen_exit_tb(0);
             break;
         case DISAS_WFE:
             gen_helper_wfe(cpu_env);