diff --git a/hw/pci-host/designware.c b/hw/pci-host/designware.c
index dd9e389c07..c25d50f1c6 100644
--- a/hw/pci-host/designware.c
+++ b/hw/pci-host/designware.c
@@ -340,6 +340,8 @@ static void designware_pcie_root_config_write(PCIDevice *d, uint32_t address,
         break;
 
     case DESIGNWARE_PCIE_ATU_VIEWPORT:
+        val &= DESIGNWARE_PCIE_ATU_REGION_INBOUND |
+                (DESIGNWARE_PCIE_NUM_VIEWPORTS - 1);
         root->atu_viewport = val;
         break;