Re: [PATCH v4 06/19] KVM: VMX: Set FRED MSR interception

[Sean Christopherson <seanjc@google.com>]

On Fri, Mar 28, 2025, Xin Li (Intel) wrote:
> @@ -7935,6 +7945,34 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>  
> +static void vmx_set_intercept_for_fred_msr(struct kvm_vcpu *vcpu)
> +{

This function should short-circult on

	if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
		return;

Functionally, it shouldn't matter.  It's mostly for documentation purposes, and
to avoid doing unnecessary work.

> +	bool flag = !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);

"flag" is unnecessarily ambiguous (eww, I see that the exiting PT code does that).
I like "set", as it has (hopefully) obvious polarity, and aligns with the function
being called.

> +
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP1, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP2, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP3, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_STKLVLS, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP1, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP2, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP3, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_CONFIG, MSR_TYPE_RW, flag);
> +
> +	/*
> +	 * IA32_FRED_RSP0 and IA32_PL0_SSP (a.k.a. IA32_FRED_SSP0) are only used
> +	 * for delivering events when running userspace, while KVM always runs in
> +	 * kernel mode (the CPL is always 0 after any VM exit), thus KVM can run
> +	 * safely with guest IA32_FRED_RSP0 and IA32_PL0_SSP.
> +	 *
> +	 * As a result, no need to intercept IA32_FRED_RSP0 and IA32_PL0_SSP.
> +	 *
> +	 * Note, save and restore of IA32_PL0_SSP belong to CET supervisor context
> +	 * management no matter whether FRED is enabled or not.  So leave its
> +	 * state management to CET code.
> +	 */
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP0, MSR_TYPE_RW, flag);
> +}