linux-kernel - Re: [PATCH v2 07/25] KVM: VMX: Set intercept for FRED MSRs

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Date: Wed, 12 Jun 2024 14:32:02 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Xin Li <xin3.li@...el.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	pbonzini@...hat.com, corbet@....net, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	shuah@...nel.org, vkuznets@...hat.com, peterz@...radead.org, 
	ravi.v.shankar@...el.com, xin@...or.com
Subject: Re: [PATCH v2 07/25] KVM: VMX: Set intercept for FRED MSRs

On Fri, Apr 19, 2024, Chao Gao wrote:
> On Wed, Feb 07, 2024 at 09:26:27AM -0800, Xin Li wrote:
> >Add FRED MSRs to the valid passthrough MSR list and set FRED MSRs intercept
> >based on FRED enumeration.

This needs a *much* more verbose explanation.  It's pretty darn obvious _what_
KVM is doing, but it's not at all clear _why_ KVM is passing through FRED MSRs.
E.g. why is FRED_SSP0 not included in the set of passthrough MSRs?

> > static void vmx_vcpu_config_fred_after_set_cpuid(struct kvm_vcpu *vcpu)
> > {
> > 	struct vcpu_vmx *vmx = to_vmx(vcpu);
> >+	bool fred_enumerated;
> > 
> > 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_FRED);
> >+	fred_enumerated = guest_can_use(vcpu, X86_FEATURE_FRED);
> > 
> >-	if (guest_can_use(vcpu, X86_FEATURE_FRED)) {
> >+	if (fred_enumerated) {
> > 		vm_entry_controls_setbit(vmx, VM_ENTRY_LOAD_IA32_FRED);
> > 		secondary_vm_exit_controls_setbit(vmx,
> > 						  SECONDARY_VM_EXIT_SAVE_IA32_FRED |
> >@@ -7788,6 +7793,16 @@ static void vmx_vcpu_config_fred_after_set_cpuid(struct kvm_vcpu *vcpu)
> > 						    SECONDARY_VM_EXIT_SAVE_IA32_FRED |
> > 						    SECONDARY_VM_EXIT_LOAD_IA32_FRED);
> > 	}
> >+
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP0, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP1, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP2, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP3, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_STKLVLS, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP1, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP2, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP3, MSR_TYPE_RW, !fred_enumerated);
> >+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_CONFIG, MSR_TYPE_RW, !fred_enumerated);
> 
> Use a for-loop here? e.g., 
> 	for (i = MSR_IA32_FRED_RSP0; i <= MSR_IA32_FRED_CONFIG; i++)

Hmm, I'd prefer to keep the open coded version.  It's not pretty, but I don't
expect this to have much, if any, maintenance cost.  And using a loop makes it
harder to both understand _exactly_ what's happening, and to search for relevant
code.  E.g. it's quite difficult to see that FRED_SSP0 is still intercepted (see
my comment regarding the changelog).