linux-kernel - Re: [Patch v3 03/13] x86/speculation: Add static key for Enhanced IBRS

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Message-ID: <alpine.DEB.2.21.1810281031520.2471@nanos.tec.linutronix.de>
Date:   Sun, 28 Oct 2018 10:32:38 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Waiman Long <longman9394@...il.com>,
        Jiri Kosina <jikos@...nel.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>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [Patch v3 03/13] x86/speculation: Add static key for Enhanced
 IBRS

On Fri, 26 Oct 2018, Tim Chen wrote:
> On 10/26/2018 09:58 AM, Waiman Long wrote:
> >> @@ -386,6 +389,7 @@ static void __init spectre_v2_select_mitigation(void)
> >>  			/* Force it so VMEXIT will restore correctly */
> >>  			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> >>  			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> >> +			static_branch_enable(&spectre_v2_enhanced_ibrs);
> >>  			goto specv2_set_mode;
> >>  		}
> >>  		if (IS_ENABLED(CONFIG_RETPOLINE))
> > 
> > Why you need a static key for enhanced IBRS? It is supposed to be set at
> > boot time and never get changed after that. It will be easier to use a
> > feature bit for that instead. We usually use static key when the value
> > can be changed at run time.
> >
> 
> We're close to running out of the feature bits.  So I'm trying not to
> use those.

Software feature bits can be extended when needed. That's really a non issue.

Thanks,

	tglx