KVM: s390: Do not yield when target is already running

If the target is already running we do not need to yield.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>

arch/s390/kvm/diag.c

@@ -168,6 +168,10 @@ static int __diag_time_slice_end_directed(struct kvm_vcpu *vcpu)
 	if (!tcpu)
 		goto no_yield;
 
+	/* target already running */
+	if (READ_ONCE(tcpu->cpu) >= 0)
+		goto no_yield;
+
 	if (kvm_vcpu_yield_to(tcpu) <= 0)
 		goto no_yield;