diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
index b7de23c7d2..652e1712ec 100644
--- a/hw/s390x/s390-pci-inst.c
+++ b/hw/s390x/s390-pci-inst.c
@@ -655,6 +655,7 @@ static uint32_t s390_pci_update_iotlb(S390PCIIOMMU *iommu,
         goto out;
     } else {
         if (cache) {
+            /* valid->valid transitions reuse the DMA slot */
             if (cache->perm == entry->perm &&
                 cache->translated_addr == entry->translated_addr) {
                 goto out;
@@ -665,6 +666,9 @@ static uint32_t s390_pci_update_iotlb(S390PCIIOMMU *iommu,
             memory_region_notify_iommu(&iommu->iommu_mr, 0, event);
             event.type = IOMMU_NOTIFIER_MAP;
             event.entry.perm = entry->perm;
+        } else {
+            /* invalid->valid transitions consume a new DMA slot */
+            dec_dma_avail(iommu);
         }
 
         cache = g_new(S390IOTLBEntry, 1);
@@ -673,7 +677,6 @@ static uint32_t s390_pci_update_iotlb(S390PCIIOMMU *iommu,
         cache->len = TARGET_PAGE_SIZE;
         cache->perm = entry->perm;
         g_hash_table_replace(iommu->iotlb, &cache->iova, cache);
-        dec_dma_avail(iommu);
     }
 
     /*