Date: Fri, 12 Jan 2018 14:57:14 -0800
From: Andi Kleen <ak@linux.intel.com>
To: Dominik Brodowski <law@dominikbrodowski.net>
Cc: Andi Kleen <andi@firstfloor.org>, tglx@linutronix.de,
        x86@kernel.org, dwmw@amazon.co.uk, linux-kernel@vger.kernel.org,
        pjt@google.com, torvalds@linux-foundation.org,
        gregkh@linux-foundation.org, peterz@infradead.org,
        luto@amacapital.net, thomas.lendacky@amd.com,
        arjan.van.de.ven@intel.com
Subject: Re: [PATCH 1/4] x86/retpoline: Add new mode RETPOLINE_UNDERFLOW
Message-ID: <20180112225714.GQ6718@tassilo.jf.intel.com>
References: <20180112184550.6573-1-andi@firstfloor.org>
 <20180112184550.6573-2-andi@firstfloor.org>
 <20180112221313.GA13035@light.dominikbrodowski.net>
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On Fri, Jan 12, 2018 at 11:13:13PM +0100, Dominik Brodowski wrote:
> > @@ -209,6 +234,11 @@ static void __init spectre_v2_select_mitigation(void)
> >  		mode = retp_compiler() ? SPECTRE_V2_RETPOLINE_GENERIC :
> >  					 SPECTRE_V2_RETPOLINE_MINIMAL;
> >  		setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
> > +		if (mode == SPECTRE_V2_RETPOLINE_GENERIC &&
> > +			cpu_needs_underflow_protection()) {
> > +			mode = SPECTRE_V2_RETPOLINE_UNDERFLOW;
> > +			setup_force_cpu_cap(X86_FEATURE_RETURN_UNDERFLOW);
> > +		}
> >  	}
> 
> Why does this require a retp-compatible compiler to be enabled?

Without the compiler you have a lot of other problems already,
so it's fairly pointless.

-Andi