KVM: x86: Update comments about MSR lists exposed to userspace

Refresh comments about msrs_to_save, emulated_msrs, and msr_based_features
to remove stale references left behind by commit 2374b731 (KVM:
x86/pmu: Use separate array for defining "PMU MSRs to save"), and to
better reflect the current reality, e.g. emulated_msrs is no longer just
for MSRs that are "kvm-specific".
Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/r/20230607004636.1421424-1-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -1423,15 +1423,14 @@ int kvm_emulate_rdpmc(struct kvm_vcpu *vcpu)
 EXPORT_SYMBOL_GPL(kvm_emulate_rdpmc);
 
 /*
- * List of msr numbers which we expose to userspace through KVM_GET_MSRS
- * and KVM_SET_MSRS, and KVM_GET_MSR_INDEX_LIST.
- *
- * The three MSR lists(msrs_to_save, emulated_msrs, msr_based_features)
- * extract the supported MSRs from the related const lists.
- * msrs_to_save is selected from the msrs_to_save_all to reflect the
- * capabilities of the host cpu. This capabilities test skips MSRs that are
- * kvm-specific. Those are put in emulated_msrs_all; filtering of emulated_msrs
- * may depend on host virtualization features rather than host cpu features.
+ * The three MSR lists(msrs_to_save, emulated_msrs, msr_based_features) track
+ * the set of MSRs that KVM exposes to userspace through KVM_GET_MSRS,
+ * KVM_SET_MSRS, and KVM_GET_MSR_INDEX_LIST.  msrs_to_save holds MSRs that
+ * require host support, i.e. should be probed via RDMSR.  emulated_msrs holds
+ * MSRs that KVM emulates without strictly requiring host support.
+ * msr_based_features holds MSRs that enumerate features, i.e. are effectively
+ * CPUID leafs.  Note, msr_based_features isn't mutually exclusive with
+ * msrs_to_save and emulated_msrs.
  */
 
 static const u32 msrs_to_save_base[] = {
@@ -1527,11 +1526,11 @@ static const u32 emulated_msrs_all[] = {
 	MSR_IA32_UCODE_REV,
 
 	/*
-	 * The following list leaves out MSRs whose values are determined
-	 * by arch/x86/kvm/vmx/nested.c based on CPUID or other MSRs.
-	 * We always support the "true" VMX control MSRs, even if the host
-	 * processor does not, so I am putting these registers here rather
-	 * than in msrs_to_save_all.
+	 * KVM always supports the "true" VMX control MSRs, even if the host
+	 * does not.  The VMX MSRs as a whole are considered "emulated" as KVM
+	 * doesn't strictly require them to exist in the host (ignoring that
+	 * KVM would refuse to load in the first place if the core set of MSRs
+	 * aren't supported).
 	 */
 	MSR_IA32_VMX_BASIC,
 	MSR_IA32_VMX_TRUE_PINBASED_CTLS,