KVM: x86/pmu: Explicitly check for RDPMC of unsupported Intel PMC types

Explicitly check for attempts to read unsupported PMC types instead of
letting the bounds check fail.  Functionally, letting the check fail is
ok, but it's unnecessarily subtle and does a poor job of documenting the
architectural behavior that KVM is emulating.

Reviewed-by: Dapeng Mi  <dapeng1.mi@linux.intel.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/vmx/pmu_intel.c

@@ -26,6 +26,7 @@
  * further confuse things, non-architectural PMUs use bit 31 as a flag for
  * "fast" reads, whereas the "type" is an explicit value.
  */
+#define INTEL_RDPMC_GP		0
 #define INTEL_RDPMC_FIXED	INTEL_PMC_FIXED_RDPMC_BASE
 
 #define INTEL_RDPMC_TYPE_MASK	GENMASK(31, 16)
@@ -89,21 +90,29 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
 		return NULL;
 
 	/*
-	 * Fixed PMCs are supported on all architectural PMUs.  Note, KVM only
-	 * emulates fixed PMCs for PMU v2+, but the flag itself is still valid,
-	 * i.e. let RDPMC fail due to accessing a non-existent counter.
+	 * General Purpose (GP) PMCs are supported on all PMUs, and fixed PMCs
+	 * are supported on all architectural PMUs, i.e. on all virtual PMUs
+	 * supported by KVM.  Note, KVM only emulates fixed PMCs for PMU v2+,
+	 * but the type itself is still valid, i.e. let RDPMC fail due to
+	 * accessing a non-existent counter.  Reject attempts to read all other
+	 * types, which are unknown/unsupported.
 	 */
-	idx &= ~INTEL_RDPMC_FIXED;
-	if (type == INTEL_RDPMC_FIXED) {
+	switch (type) {
+	case INTEL_RDPMC_FIXED:
 		counters = pmu->fixed_counters;
 		num_counters = pmu->nr_arch_fixed_counters;
 		bitmask = pmu->counter_bitmask[KVM_PMC_FIXED];
-	} else {
+		break;
+	case INTEL_RDPMC_GP:
 		counters = pmu->gp_counters;
 		num_counters = pmu->nr_arch_gp_counters;
 		bitmask = pmu->counter_bitmask[KVM_PMC_GP];
+		break;
+	default:
+		return NULL;
 	}
 
+	idx &= INTEL_RDPMC_INDEX_MASK;
 	if (idx >= num_counters)
 		return NULL;