Re: [PATCH v10 22/27] KVM: VMX: Set up interception for CET MSRs

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Sun, Feb 18, 2024, Yang Weijiang wrote:
> @@ -7767,6 +7771,41 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>  
> +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
> +{
> +	bool incpt;
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> +
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
> +					  MSR_TYPE_RW, incpt);
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> +					  MSR_TYPE_RW, incpt);
> +		if (!incpt)
> +			return;

Hmm, I find this is unnecessarily confusing and brittle.  E.g. in the unlikely
event more CET stuff comes along, this lurking return could cause problems.

Why not handle S_CET and U_CET in a single common path?  IMO, this is less error
prone, and more clearly captures the relationship between S/U_CET, SHSTK, and IBT.
Updating MSR intercepts is not a hot path, so the overhead of checking guest CPUID
multiple times should be a non-issue.  And eventually KVM should effectively cache
all of those lookups, i.e. the cost will be negilible.

	bool incpt;

	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);

		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
					  MSR_TYPE_RW, incpt);
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
					  MSR_TYPE_RW, incpt);
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
					  MSR_TYPE_RW, incpt);
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
					  MSR_TYPE_RW, incpt);
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
					  MSR_TYPE_RW, incpt);
	}

	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
	    kvm_cpu_cap_has(X86_FEATURE_IBT)) {
		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT) &&
			!guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);

		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
					  MSR_TYPE_RW, incpt);
		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
					  MSR_TYPE_RW, incpt);
	}