Re: [Xen-devel] [PATCH 9/9] x86/amd: Offer MSR_VIRT_SPEC_CTRL to guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.18 at 17:18, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/cpuid.c
> +++ b/xen/arch/x86/cpuid.c
> @@ -370,6 +370,16 @@ static void __init 
> guest_common_feature_adjustments(uint32_t *fs)
>       */
>      if ( host_cpuid_policy.feat.ibrsb )
>          __set_bit(X86_FEATURE_IBPB, fs);
> +
> +    /*
> +     * In practice, we can offer VIRT_SC_SSBD on any hardware with 
> legacy_ssbd
> +     * or msr_spec_ctrl, but until we've got a proper split between default
> +     * and max policies, avoid offering it in cases where the guest shouldn't
> +     * be using it.
> +     */
> +    __clear_bit(X86_FEATURE_VIRT_SC_SSBD, fs);
> +    if ( cpu_has_legacy_ssbd )
> +        __set_bit(X86_FEATURE_VIRT_SC_SSBD, fs);
>  }

Is the comment really correct wrt msr_spec_ctrl? The MSR can
exist without there being support for SSBD, can't it?

Jan



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