linux-kernel - Re: [Patch v5 11/16] x86/speculation: Add Spectre v2 app to app protection modes

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Date:   Mon, 19 Nov 2018 21:55:27 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        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>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [Patch v5 11/16] x86/speculation: Add Spectre v2 app to app
 protection modes

On Mon, 19 Nov 2018, Thomas Gleixner wrote:

> > @@ -452,12 +542,6 @@ static void __init spectre_v2_select_mitigation(void)
> >  	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> >  	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
> >  
> > -	/* Initialize Indirect Branch Prediction Barrier if supported */
> > -	if (boot_cpu_has(X86_FEATURE_IBPB)) {
> > -		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
> > -		pr_info("Spectre v2 mitigation: Enabling Indirect Branch Prediction Barrier\n");
> > -	}
> > -
> >  	/*
> >  	 * Retpoline means the kernel is safe because it has no indirect
> >  	 * branches. Enhanced IBRS protects firmware too, so, enable restricted
> > @@ -474,6 +558,43 @@ static void __init spectre_v2_select_mitigation(void)
> >  		pr_info("Enabling Restricted Speculation for firmware calls\n");
> >  	}
> >  
> > +	app2app_mode = SPECTRE_V2_APP2APP_NONE;
> > +	if (!boot_cpu_has(X86_FEATURE_IBPB) ||
> > +	    !boot_cpu_has(X86_FEATURE_STIBP))
> > +		goto set_app2app_mode;
> 
> So before that change IBPB was usable without STIBP, now not longer. What's
> the rationale?
> 
> This patch changes a gazillion things at once and is completely
> unreviewable.

The patchset actually ties together IBPB and STIBP pretty closely, which 
is IMO a good thing; there is no good reason why anone would want just one 
of those (or each in a different mode), at least before this magical 
coscheduling exists.

But I guess this fact should be documented somewhere.

-- 
Jiri Kosina
SUSE Labs