Re: [RFC PATCH v2 6/6] KVM: SVM: Add CET features to supported_xss

[Edgecombe, Rick P]

On Wed, 2023-05-24 at 15:53 +0000, John Allen wrote:
> If the CPU supports CET, add CET XSAVES feature bits to the
> supported_xss mask.
> 
> Signed-off-by: John Allen <john.allen@xxxxxxx>
> ---
> v2:
>   - Remove curly braces around if statement
> ---
>  arch/x86/kvm/svm/svm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 6afd2c44fdb6..cee496bee0a9 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5070,6 +5070,10 @@ static __init void svm_set_cpu_caps(void)
>             boot_cpu_has(X86_FEATURE_AMD_SSBD))
>                 kvm_cpu_cap_set(X86_FEATURE_VIRT_SSBD);
>  
> +       if (kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> +               kvm_caps.supported_xss |= XFEATURE_MASK_CET_USER |
> +                                         XFEATURE_MASK_CET_KERNEL;
> +

Is setting XFEATURE_MASK_CET_KERNEL here ok? The host kernel will not
support XFEATURE_MASK_CET_KERNEL. I guess after this there is a small
window of time where host IA32_XSS could have non-host supported
supervisor state.

Sort of separately, how does SVM work with respect to saving and
restoring guest supervisor CET state (I mean the CET_S stuff)?

I'm not sure there is any problem, but just wondering how it all works.
Thanks.