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

From: Thomas Gleixner <tglx@linutronix.de>
To: Jiri Kosina <jikos@kernel.org>
Cc: Tim Chen <tim.c.chen@linux.intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Casey Schaufler <casey.schaufler@intel.com>,
	Asit Mallick <asit.k.mallick@intel.com>,
	Arjan van de Ven <arjan@linux.intel.com>,
	Jon Masters <jcm@redhat.com>, Waiman Long <longman9394@gmail.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [Patch v5 11/16] x86/speculation: Add Spectre v2 app to app protection modes
Date: Mon, 19 Nov 2018 22:12:44 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.21.1811192212070.1669@nanos.tec.linutronix.de> (raw)
In-Reply-To: <nycvar.YFH.7.76.1811192153470.21108@cbobk.fhfr.pm>

On Mon, 19 Nov 2018, Jiri Kosina wrote:
> 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.

That and it can be split in pieces so it actually becomes reviewable.

Thanks,

	tglx