linux-kernel - Re: [PATCH 1/4] x86/retpoline: Add new mode RETPOLINE

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Date:   Fri, 12 Jan 2018 14:57:14 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Dominik Brodowski <law@...inikbrodowski.net>
Cc:     Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
        x86@...nel.org, dwmw@...zon.co.uk, linux-kernel@...r.kernel.org,
        pjt@...gle.com, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, peterz@...radead.org,
        luto@...capital.net, thomas.lendacky@....com,
        arjan.van.de.ven@...el.com
Subject: Re: [PATCH 1/4] x86/retpoline: Add new mode RETPOLINE_UNDERFLOW

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