KVM: arm64: Fix AArch32 PMUv3 capping

We shouldn't expose *any* PMU capability when no PMU has been
configured for this VM.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/sys_regs.c

@@ -1048,8 +1048,8 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
 	} else if (id == SYS_ID_DFR0_EL1) {
 		/* Limit guests to PMUv3 for ARMv8.1 */
 		val = cpuid_feature_cap_perfmon_field(val,
-						ID_DFR0_PERFMON_SHIFT,
-						ID_DFR0_PERFMON_8_1);
+						      ID_DFR0_PERFMON_SHIFT,
+						      kvm_vcpu_has_pmu(vcpu) ? ID_DFR0_PERFMON_8_1 : 0);
 	}
 
 	return val;