Re: [PATCH v15 16/41] KVM: VMX: Set up interception for CET MSRs

[Binbin Wu]

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>
> Enable/disable CET MSRs interception per associated feature configuration.
>
> Pass through CET MSRs that are managed by XSAVE, as they cannot be
> intercepted without also intercepting XSAVE. However, intercepting XSAVE
> would likely cause unacceptable performance overhead.
Here may be a bit confusing about the description of "managed by XSAVE" because
KVM has a function is_xstate_managed_msr(), and MSR_IA32_S_CET is not xstate
managed in it.

Otherwise,
Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> MSR_IA32_INT_SSP_TAB is not managed by XSAVE, so it is intercepted.
>
> Note, this MSR design introduced an architectural limitation of SHSTK and
> IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
> to guest from architectural perspective since IBT relies on subset of SHSTK
> relevant MSRs.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> Tested-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4fc1dbba2eb0..adf5af30e537 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4101,6 +4101,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>   
>   void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>   {
> +	bool intercept;
> +
>   	if (!cpu_has_vmx_msr_bitmap())
>   		return;
>   
> @@ -4146,6 +4148,23 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>   		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
>   					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
>   
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> +		intercept = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, intercept);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, intercept);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, intercept);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, intercept);
> +	}
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) || kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> +		intercept = !guest_cpu_cap_has(vcpu, X86_FEATURE_IBT) &&
> +			    !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, intercept);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, intercept);
> +	}
> +
>   	/*
>   	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
>   	 * filtered by userspace.