Re: [PATCH] x86/retpoline: Also fill return buffer after idle

[Peter Zijlstra]

On Mon, Jan 08, 2018 at 03:51:26PM -0800, Andi Kleen wrote:

> @@ -107,8 +109,15 @@ static inline void mwait_idle_with_hints(unsigned long 
> eax, unsigned long ecx)
>               }
>  
>               __monitor((void *)&current_thread_info()->flags, 0, 0);
> -             if (!need_resched())
> +             if (!need_resched()) {
>                       __mwait(eax, ecx);
> +                     /*
> +                      * idle could have cleared the return buffer,
> +                      * so fill it to prevent uncontrolled
> +                      * speculation.
> +                      */
> +                     fill_return_buffer();

wouldn't something like:

                        if (static_cpu_has(X86_FEATURE_RETPOLINE))
                                fill_return_buffer();

be much saner? Then we avoid the entire call when not needed and you
don't have to muck with the asm either.

Also, you forgot mwait_idle() in process.c

> +             }
>       }
>       current_clr_polling();
>  }
> -- 
> 2.14.3
>