Re: [Xen-devel] [PATCH v9 07/11] x86/boot: Calculate the most appropriate BTI mitigation to use

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.01.18 at 16:46, <andrew.cooper3@xxxxxxxxxx> wrote:
> @@ -124,7 +186,21 @@ void __init init_speculation_mitigations(void)
>               */
>              if ( cpu_has_lfence_dispatch )
>                  thunk = THUNK_LFENCE;
> +            /*
> +             * On Intel hardware, we'd like to use retpoline in preference to
> +             * IBRS, but only if it is safe on this hardware.
> +             */
> +            else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
> +            {
> +                if ( retpoline_safe() )
> +                    thunk = THUNK_RETPOLINE;
> +                else
> +                    ibrs = true;
> +            }

I think I had asked about this piece of code before, but maybe not
the same I'm noticing now: Why is using retpoline dependent upon
IBRSB? I.e. why not

            else if ( retpoline_safe() )
                thunk = THUNK_RETPOLINE;
            else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
                ibrs = true;

?

Jan


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