Re: [PATCH 1/4] x86/retpoline: Add new mode RETPOLINE_UNDERFLOW

[Dominik Brodowski]

> @@ -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