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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