KVM: PPC: Be more informative on BUG

We have a condition in the ppc64 host mmu code that should never occur.
Unfortunately, it just did happen to me and I was rather puzzled on why,
because BUG_ON doesn't tell me anything useful.

So let's add some more debug output in case this goes wrong. Also change
BUG to WARN, since I don't want to reboot every time I mess something up.
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/powerpc/kvm/book3s_64_mmu_host.c

@@ -231,10 +231,16 @@ int kvmppc_mmu_map_page(struct kvm_vcpu *vcpu, struct kvmppc_pte *orig_pte)
 	vcpu->arch.mmu.esid_to_vsid(vcpu, orig_pte->eaddr >> SID_SHIFT, &vsid);
 	map = find_sid_vsid(vcpu, vsid);
 	if (!map) {
-		kvmppc_mmu_map_segment(vcpu, orig_pte->eaddr);
+		ret = kvmppc_mmu_map_segment(vcpu, orig_pte->eaddr);
+		WARN_ON(ret < 0);
 		map = find_sid_vsid(vcpu, vsid);
 	}
-	BUG_ON(!map);
+	if (!map) {
+		printk(KERN_ERR "KVM: Segment map for 0x%llx (0x%lx) failed\n",
+				vsid, orig_pte->eaddr);
+		WARN_ON(true);
+		return -EINVAL;
+	}
 
 	vsid = map->host_vsid;
 	va = hpt_va(orig_pte->eaddr, vsid, MMU_SEGSIZE_256M);