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

[Peter Zijlstra <peterz@infradead.org>]

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
>