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

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Date:   Tue, 9 Jan 2018 10:37:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     dwmw2@...radead.org, pjt@...gle.com, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        tim.c.chen@...ux.intel.com, dave.hansen@...el.com,
        tglx@...utronix.de, luto@...capital.net,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] x86/retpoline: Also fill return buffer after idle

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
>