linux-kernel - Re: [PATCH v5 2/2] x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation

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Message-ID: <alpine.DEB.2.21.1809112344050.1427@nanos.tec.linutronix.de>
Date:   Tue, 11 Sep 2018 23:46:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Jiri Kosina <jikos@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v5 2/2] x86/speculation: Enable cross-hyperthread spectre
 v2 STIBP mitigation

On Tue, 11 Sep 2018, Thomas Gleixner wrote:

> On Tue, 11 Sep 2018, Tim Chen wrote:
> > On 09/10/2018 04:46 AM, Jiri Kosina wrote:
> > > Nah, IBPB is actuall there, sorry. So I'll add reporting of STIBP + fixup 
> > > the missing reporting of RSB_CTXSW for v6.
> > > 
> > 
> > I anticipate that STIBP could affect workloads with a lot of indirect
> > branches (see previous discussion with Andrea).  We should have a 
> > knob for people to opt in or opt out of STIBP.
> 
> Feel free to send a patch to that effect.

Along with a proper explanation why it makes sense to disable this
particular piece separate from the rest of the V2 mitigations. You surely
can come up with a rationale based on facts rather than on anticipations.

Thanks,

	tglx