KVM: arm64: Make AMU sysreg UNDEF if FEAT_AMU is not advertised to the guest

No AMU? No AMU! IF we see an AMU-related trap, let's turn it into
an UNDEF!
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240214131827.2856277-24-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -3998,6 +3998,10 @@ void kvm_init_sysreg(struct kvm_vcpu *vcpu)
 		kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nPIRE0_EL1 |
 						HFGxTR_EL2_nPIR_EL1);
 
+	if (!kvm_has_feat(kvm, ID_AA64PFR0_EL1, AMU, IMP))
+		kvm->arch.fgu[HAFGRTR_GROUP] |= ~(HAFGRTR_EL2_RES0 |
+						  HAFGRTR_EL2_RES1);
+
 	set_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags);
 out:
 	mutex_unlock(&kvm->arch.config_lock);