Let's prepare our address space for memory devices if enabled via
"maxmem" and if we have CONFIG_MEM_DEVICE enabled at all. Note that
CONFIG_MEM_DEVICE will be selected automatically once we add support
for devices.

Just like on other architectures, the region container for memory devices
is placed directly above our initial memory. For now, we only align the
start address of the region up to 1 GiB, but we won't add any additional
space to the region for internal alignment purposes; this can be done in
the future if really required.

The RAM size returned via SCLP is not modified, as this only
covers initial RAM (and standby memory we don't implement) and not memory
devices; clarify that in the docs of read_SCP_info(). Existing OSes without
support for memory devices will keep working as is, even when memory
devices would be attached the VM.

Guest OSs which support memory devices, such as virtio-mem, will
consult diag500(), to find out the maximum possible pfn. Guest OSes that
don't support memory devices, don't have to be changed and will continue
relying on information provided by SCLP.

There are no remaining maxram_size users in s390x code, and the remaining
ram_size users only care about initial RAM:
* hw/s390x/ipl.c
* hw/s390x/s390-hypercall.c
* hw/s390x/sclp.c
* target/s390x/kvm/pv.c

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/s390x/s390-virtio-ccw.c | 25 +++++++++++++++++++++++--
 hw/s390x/sclp.c            |  6 +++++-
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 08156f0682..0a9d25620d 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -126,7 +126,7 @@ static void s390_memory_init(MachineState *machine)
     MemoryRegion *sysmem = get_system_memory();
     MemoryRegion *ram = machine->ram;
     uint64_t ram_size = memory_region_size(ram);
-    uint64_t hw_limit;
+    uint64_t hw_limit, devmem_base, devmem_size;
     int ret;
 
     if (!QEMU_IS_ALIGNED(ram_size, 1 * MiB)) {
@@ -140,7 +140,21 @@ static void s390_memory_init(MachineState *machine)
         exit(EXIT_FAILURE);
     }
 
-    ret = s390_set_memory_limit(ram_size, &hw_limit);
+    devmem_size = 0;
+    devmem_base = ram_size;
+#ifdef CONFIG_MEM_DEVICE
+    if (machine->ram_size < machine->maxram_size) {
+
+        /*
+         * Make sure memory devices have a sane default alignment, even
+         * when weird initial memory sizes are specified.
+         */
+        devmem_base = QEMU_ALIGN_UP(devmem_base, 1 * GiB);
+        devmem_size = machine->maxram_size - machine->ram_size;
+    }
+#endif
+
+    ret = s390_set_memory_limit(devmem_base + devmem_size, &hw_limit);
     if (ret == -E2BIG) {
         error_report("host supports a maximum of %" PRIu64 " GB",
                      hw_limit / GiB);
@@ -153,6 +167,13 @@ static void s390_memory_init(MachineState *machine)
     /* Map the initial memory. Must happen after setting the memory limit. */
     memory_region_add_subregion(sysmem, 0, ram);
 
+    /* Initialize address space for memory devices. */
+#ifdef CONFIG_MEM_DEVICE
+    if (devmem_size) {
+        machine_memory_devices_init(machine, devmem_base, devmem_size);
+    }
+#endif /* CONFIG_MEM_DEVICE */
+
     /*
      * Configure the maximum page size. As no memory devices were created
      * yet, this is the page size of initial memory only.
diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
index fac09816bf..fe4216a10d 100644
--- a/hw/s390x/sclp.c
+++ b/hw/s390x/sclp.c
@@ -162,7 +162,11 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
         read_info->rnsize2 = cpu_to_be32(rnsize);
     }
 
-    /* we don't support standby memory, maxram_size is never exposed */
+    /*
+     * We don't support standby memory. maxram_size is used for sizing the
+     * memory device region, which is not exposed through SCLP but through
+     * diag500.
+     */
     rnmax = machine->ram_size >> sclp->increment_size;
     if (rnmax < 0x10000) {
         read_info->rnmax = cpu_to_be16(rnmax);
-- 
2.46.0