KVM: nSVM: Add a comment to document why nNPT uses vmcb01, not vCPU state

Add a comment in the nested NPT initialization flow to call out that it
intentionally uses vmcb01 instead current vCPU state to get the effective
hCR4 and hEFER for L1's NPT context.

Note, despite nSVM's efforts to handle the case where vCPU state doesn't
reflect L1 state, the MMU may still do the wrong thing due to pulling
state from the vCPU instead of the passed in CR0/CR4/EFER values.  This
will be addressed in future commits.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/nested.c

@@ -98,6 +98,12 @@ static void nested_svm_init_mmu_context(struct kvm_vcpu *vcpu)
 	WARN_ON(mmu_is_nested(vcpu));
 
 	vcpu->arch.mmu = &vcpu->arch.guest_mmu;
+
+	/*
+	 * The NPT format depends on L1's CR4 and EFER, which is in vmcb01.  Note,
+	 * when called via KVM_SET_NESTED_STATE, that state may _not_ match current
+	 * vCPU state.  CR0.WP is explicitly ignored, while CR0.PG is required.
+	 */
 	kvm_init_shadow_npt_mmu(vcpu, X86_CR0_PG, svm->vmcb01.ptr->save.cr4,
 				svm->vmcb01.ptr->save.efer,
 				svm->nested.ctl.nested_cr3);