KVM: PPC: Fix sid map search after flush

After a flush the sid map contained lots of entries with 0 for their gvsid and
hvsid value. Unfortunately, 0 can be a real value the guest searches for when
looking up a vsid so it would incorrectly find the host's 0 hvsid mapping which
doesn't belong to our sid space.

So let's also check for the valid bit that indicated that the sid we're
looking at actually contains useful data.
Signed-off-by: Alexander Graf <agraf@suse.de>

arch/powerpc/kvm/book3s_64_mmu_host.c

@@ -65,14 +65,14 @@ static struct kvmppc_sid_map *find_sid_vsid(struct kvm_vcpu *vcpu, u64 gvsid)
 
 	sid_map_mask = kvmppc_sid_hash(vcpu, gvsid);
 	map = &to_book3s(vcpu)->sid_map[sid_map_mask];
-	if (map->guest_vsid == gvsid) {
+	if (map->valid && (map->guest_vsid == gvsid)) {
 		dprintk_slb("SLB: Searching: 0x%llx -> 0x%llx\n",
 			    gvsid, map->host_vsid);
 		return map;
 	}
 
 	map = &to_book3s(vcpu)->sid_map[SID_MAP_MASK - sid_map_mask];
-	if (map->guest_vsid == gvsid) {
+	if (map->valid && (map->guest_vsid == gvsid)) {
 		dprintk_slb("SLB: Searching 0x%llx -> 0x%llx\n",
 			    gvsid, map->host_vsid);
 		return map;