Re: [PATCH 19/19] KVM: x86: Enable supervisor IBT support for guest

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 16, 2022 at 04:46:43AM -0400, Yang Weijiang wrote:
> Mainline kernel now supports supervisor IBT for kernel code,
> to make s-IBT work in guest(nested guest), pass through
> MSR_IA32_S_CET to guest(nested guest) if host kernel and KVM
> enabled IBT. Note, s-IBT can work independent to host xsaves
> support because guest MSR_IA32_S_CET can be stored/loaded from
> specific VMCS field.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
>  arch/x86/kvm/cpuid.h      |  5 +++++
>  arch/x86/kvm/vmx/nested.c |  3 +++
>  arch/x86/kvm/vmx/vmx.c    | 27 ++++++++++++++++++++++++---
>  arch/x86/kvm/x86.c        | 13 ++++++++++++-
>  4 files changed, 44 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index ac72aabba981..c67c1e2fc11a 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -230,4 +230,9 @@ static __always_inline bool guest_pv_has(struct kvm_vcpu *vcpu,
>  	return vcpu->arch.pv_cpuid.features & (1u << kvm_feature);
>  }
>  
> +static __always_inline bool cet_kernel_ibt_supported(void)
> +{
> +	return HAS_KERNEL_IBT && kvm_cpu_cap_has(X86_FEATURE_IBT);
> +}

As stated before; I would much rather it expose S_CET unconditional of
host kernel config.