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

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Message-ID: <20180112221313.GA13035@light.dominikbrodowski.net>
Date:   Fri, 12 Jan 2018 23:13:13 +0100
From:   Dominik Brodowski <law@...inikbrodowski.net>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     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, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/4] x86/retpoline: Add new mode RETPOLINE_UNDERFLOW

> @@ -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?

Thanks,
	Dominik