diff --git a/hw/s390x/css-bridge.c b/hw/s390x/css-bridge.c
index 9a7f7ee..b54ac01 100644
--- a/hw/s390x/css-bridge.c
+++ b/hw/s390x/css-bridge.c
@@ -107,6 +107,9 @@
     /* Enable hotplugging */
     qbus_set_hotplug_handler(bus, dev, &error_abort);
 
+    css_register_io_adapters(CSS_IO_ADAPTER_VIRTIO, true, false,
+                             &error_abort);
+
     return cbus;
  }
 
diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 1b242c1..c03bb20 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -47,7 +47,6 @@
     uint32_t id;
     uint8_t type;
     uint8_t isc;
-    QTAILQ_ENTRY(IoAdapter) sibling;
 } IoAdapter;
 
 typedef struct ChannelSubSys {
@@ -61,7 +60,7 @@
     uint64_t chnmon_area;
     CssImage *css[MAX_CSSID + 1];
     uint8_t default_cssid;
-    QTAILQ_HEAD(, IoAdapter) io_adapters;
+    IoAdapter *io_adapters[CSS_IO_ADAPTER_TYPE_NUMS][MAX_ISC + 1];
     QTAILQ_HEAD(, IndAddr) indicator_addresses;
 } ChannelSubSys;
 
@@ -72,7 +71,6 @@
     .do_crw_mchk = true,
     .crws_lost = false,
     .chnmon_active = false,
-    .io_adapters = QTAILQ_HEAD_INITIALIZER(channel_subsys.io_adapters),
     .indicator_addresses =
         QTAILQ_HEAD_INITIALIZER(channel_subsys.indicator_addresses),
 };
@@ -155,44 +153,67 @@
     return 0;
 }
 
-int css_register_io_adapter(CssIoAdapterType type, uint8_t isc, bool swap,
-                            bool maskable, uint32_t *id)
+uint32_t css_get_adapter_id(CssIoAdapterType type, uint8_t isc)
 {
+    if (type >= CSS_IO_ADAPTER_TYPE_NUMS || isc > MAX_ISC ||
+        !channel_subsys.io_adapters[type][isc]) {
+        return -1;
+    }
+
+    return channel_subsys.io_adapters[type][isc]->id;
+}
+
+/**
+ * css_register_io_adapters: Register I/O adapters per ISC during init
+ *
+ * @swap: an indication if byte swap is needed.
+ * @maskable: an indication if the adapter is subject to the mask operation.
+ * @errp: location to store error information.
+ */
+void css_register_io_adapters(CssIoAdapterType type, bool swap, bool maskable,
+                              Error **errp)
+{
+    uint32_t id;
+    int ret, isc;
     IoAdapter *adapter;
-    bool found = false;
-    int ret;
     S390FLICState *fs = s390_get_flic();
     S390FLICStateClass *fsc = S390_FLIC_COMMON_GET_CLASS(fs);
 
-    *id = 0;
-    QTAILQ_FOREACH(adapter, &channel_subsys.io_adapters, sibling) {
-        if ((adapter->type == type) && (adapter->isc == isc)) {
-            *id = adapter->id;
-            found = true;
-            ret = 0;
+    /*
+     * Disallow multiple registrations for the same device type.
+     * Report an error if registering for an already registered type.
+     */
+    if (channel_subsys.io_adapters[type][0]) {
+        error_setg(errp, "Adapters for type %d already registered", type);
+    }
+
+    for (isc = 0; isc <= MAX_ISC; isc++) {
+        id = (type << 3) | isc;
+        ret = fsc->register_io_adapter(fs, id, isc, swap, maskable);
+        if (ret == 0) {
+            adapter = g_new0(IoAdapter, 1);
+            adapter->id = id;
+            adapter->isc = isc;
+            adapter->type = type;
+            channel_subsys.io_adapters[type][isc] = adapter;
+        } else {
+            error_setg_errno(errp, -ret, "Unexpected error %d when "
+                             "registering adapter %d", ret, id);
             break;
         }
-        if (adapter->id >= *id) {
-            *id = adapter->id + 1;
+    }
+
+    /*
+     * No need to free registered adapters in kvm: kvm will clean up
+     * when the machine goes away.
+     */
+    if (ret) {
+        for (isc--; isc >= 0; isc--) {
+            g_free(channel_subsys.io_adapters[type][isc]);
+            channel_subsys.io_adapters[type][isc] = NULL;
         }
     }
-    if (found) {
-        goto out;
-    }
-    adapter = g_new0(IoAdapter, 1);
-    ret = fsc->register_io_adapter(fs, *id, isc, swap, maskable);
-    if (ret == 0) {
-        adapter->id = *id;
-        adapter->isc = isc;
-        adapter->type = type;
-        QTAILQ_INSERT_TAIL(&channel_subsys.io_adapters, adapter, sibling);
-    } else {
-        g_free(adapter);
-        fprintf(stderr, "Unexpected error %d when registering adapter %d\n",
-                ret, *id);
-    }
-out:
-    return ret;
+
 }
 
 static void css_clear_io_interrupt(uint16_t subchannel_id,
diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
index 1570f2d..a8a1bab 100644
--- a/hw/s390x/s390-pci-bus.c
+++ b/hw/s390x/s390-pci-bus.c
@@ -581,6 +581,9 @@
     s->bus_no = 0;
     QTAILQ_INIT(&s->pending_sei);
     QTAILQ_INIT(&s->zpci_devs);
+
+    css_register_io_adapters(CSS_IO_ADAPTER_PCI, true, false, &error_abort);
+
     return 0;
 }
 
diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
index 93980d6..314a9cb 100644
--- a/hw/s390x/s390-pci-inst.c
+++ b/hw/s390x/s390-pci-inst.c
@@ -733,12 +733,10 @@
 static int reg_irqs(CPUS390XState *env, S390PCIBusDevice *pbdev, ZpciFib fib)
 {
     int ret, len;
+    uint8_t isc = FIB_DATA_ISC(ldl_p(&fib.data));
 
-    ret = css_register_io_adapter(CSS_IO_ADAPTER_PCI,
-                                  FIB_DATA_ISC(ldl_p(&fib.data)), true, false,
-                                  &pbdev->routes.adapter.adapter_id);
-    assert(ret == 0);
-
+    pbdev->routes.adapter.adapter_id = css_get_adapter_id(
+                                       CSS_IO_ADAPTER_PCI, isc);
     pbdev->summary_ind = get_indicator(ldq_p(&fib.aisb), sizeof(uint64_t));
     len = BITS_TO_LONGS(FIB_DATA_NOI(ldl_p(&fib.data))) * sizeof(unsigned long);
     pbdev->indicator = get_indicator(ldq_p(&fib.aibv), len);
@@ -757,7 +755,7 @@
     pbdev->routes.adapter.summary_offset = FIB_DATA_AISBO(ldl_p(&fib.data));
     pbdev->routes.adapter.ind_addr = ldq_p(&fib.aibv);
     pbdev->routes.adapter.ind_offset = FIB_DATA_AIBVO(ldl_p(&fib.data));
-    pbdev->isc = FIB_DATA_ISC(ldl_p(&fib.data));
+    pbdev->isc = isc;
     pbdev->noi = FIB_DATA_NOI(ldl_p(&fib.data));
     pbdev->sum = FIB_DATA_SUM(ldl_p(&fib.data));
 
diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index b728439..f376381 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -616,10 +616,9 @@
                 dev->routes.adapter.ind_offset = ind_bit;
                 dev->routes.adapter.summary_offset = 7;
                 cpu_physical_memory_unmap(thinint, hw_len, 0, hw_len);
-                ret = css_register_io_adapter(CSS_IO_ADAPTER_VIRTIO,
-                                              dev->thinint_isc, true, false,
-                                              &dev->routes.adapter.adapter_id);
-                assert(ret == 0);
+                dev->routes.adapter.adapter_id = css_get_adapter_id(
+                                                 CSS_IO_ADAPTER_VIRTIO,
+                                                 dev->thinint_isc);
                 sch->thinint_active = ((dev->indicators != NULL) &&
                                        (dev->summary_indicator != NULL));
                 sch->curr_status.scsw.count = ccw.count - len;
@@ -1308,9 +1307,9 @@
     dev->thinint_isc = qemu_get_byte(f);
     dev->revision = qemu_get_be32(f);
     if (s->thinint_active) {
-        return css_register_io_adapter(CSS_IO_ADAPTER_VIRTIO,
-                                       dev->thinint_isc, true, false,
-                                       &dev->routes.adapter.adapter_id);
+        dev->routes.adapter.adapter_id = css_get_adapter_id(
+                                         CSS_IO_ADAPTER_VIRTIO,
+                                         dev->thinint_isc);
     }
 
     return 0;