Re: [Qemu-devel] [PATCH 13/14] target-arm: Don't halt on WFI unless we don't have any work

["Edgar E. Iglesias" <edgar.iglesias@xilinx.com>]

On Tue, May 19, 2015 at 07:33:33PM +0100, Peter Maydell wrote:
> 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: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>


> ---
>  target-arm/op_helper.c     | 7 +++++++
>  target-arm/translate-a64.c | 4 ++++
>  target-arm/translate.c     | 4 ++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/target-arm/op_helper.c b/target-arm/op_helper.c
> index 5963f3b..517dacc 100644
> --- a/target-arm/op_helper.c
> +++ b/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);
> diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
> index 8d08ccd..ffa6cb8 100644
> --- a/target-arm/translate-a64.c
> +++ b/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;
>          }
>      }
> diff --git a/target-arm/translate.c b/target-arm/translate.c
> index ed2c43d..6493b9a 100644
> --- a/target-arm/translate.c
> +++ b/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);
> -- 
> 1.9.1
>