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

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Message-ID: <20181122065850.GA11963@kroah.com>
Date:   Thu, 22 Nov 2018 07:58:50 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        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>,
        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, Nov 21, 2018 at 09:51:19PM +0100, Jiri Kosina wrote:
> 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.

It's in the latest stable -rc releases, and if all goes well, they will
be released tomorrow.

thanks,

greg k-h