linux-kernel - Re: [PATCH] x86/speculation: Revert turning on STIBP all the time

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Date:   Wed, 21 Nov 2018 21:51:19 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] x86/speculation: Revert turning on STIBP all the time

On Wed, 21 Nov 2018, Tim Chen wrote:

> Commit 53c613fe "x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation"
> turns on STIBP all the time.
> 
> This causes large performance regression in many workloads.
> 
> One case is perlbench in the SpecInt Rate 2006 test suite which shows a
> 21% reduction in throughput.
> There're also other reports of drop in performance on Python and PHP benchmarks:
> https://www.phoronix.com/scan.php?page=article&item=linux-420-bisect&num=2
> 
> STIBP on all the time should not be the default option.
> 
> Turn off STIBP all the time for now till STIBP can be applied on
> a per task basis.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index c37e66e..21a8f39 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -332,7 +332,7 @@ static bool stibp_needed(void)
>  	if (!boot_cpu_has(X86_FEATURE_STIBP))
>  		return false;
>  
> -	return true;
> +	return false;
>  }

For -stable, which actually makes it to production, I already asked Greg 
to drop it.

For -rc, I don't think we need to do this at this moment, given the 
prctl+seccomp fixup is basically ready, do we?

Thanks,

-- 
Jiri Kosina
SUSE Labs