KVM: arm64: Provide 1 event counter on IMPDEF hardware

PMUv3 requires that all programmable event counters are capable of
counting any event. The Apple M* PMU is quite a bit different, and
events have affinities for particular PMCs.

Expose 1 event counter on IMPDEF hardware, allowing the guest to do
something useful with its PMU while also upholding the requirements of
the architecture.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: oupton

arch/arm64/kvm/pmu-emul.c

@@ -942,8 +942,12 @@ u8 kvm_arm_pmu_get_max_counters(struct kvm *kvm)
 {
 	struct arm_pmu *arm_pmu = kvm->arch.arm_pmu;
 
+	/*
+	 * PMUv3 requires that all event counters are capable of counting any
+	 * event, though the same may not be true of non-PMUv3 hardware.
+	 */
 	if (cpus_have_final_cap(ARM64_WORKAROUND_PMUV3_IMPDEF_TRAPS))
-		return 0;
+		return 1;
 
 	/*
 	 * The arm_pmu->cntr_mask considers the fixed counter(s) as well.