This device can be used to create some memories using standard
device_add qmp command.

This device has one property 'readonly' which allows
to choose between a ram or a rom.
The device holds the adequate memory region and can be put on
the sysbus.

Signed-off-by: Damien Hedde <damien.hedde@greensocs.com>
---

Should we add a related CONFIG_ variable in the build system ?

Depending on the chosen condition to add a device, the commit may
change.
---
 include/hw/mem/sysbus-memory.h | 32 +++++++++++++
 hw/mem/sysbus-memory.c         | 83 ++++++++++++++++++++++++++++++++++
 hw/mem/meson.build             |  2 +
 3 files changed, 117 insertions(+)
 create mode 100644 include/hw/mem/sysbus-memory.h
 create mode 100644 hw/mem/sysbus-memory.c

diff --git a/include/hw/mem/sysbus-memory.h b/include/hw/mem/sysbus-memory.h
new file mode 100644
index 0000000000..3e1271dbfd
--- /dev/null
+++ b/include/hw/mem/sysbus-memory.h
@@ -0,0 +1,32 @@
+/*
+ * QEMU memory SysBusDevice
+ *
+ * Copyright (c) 2021 Greensocs
+ *
+ * Author:
+ * + Damien Hedde
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef HW_SYSBUS_MEMORY_H
+#define HW_SYSBUS_MEMORY_H
+
+#include "hw/sysbus.h"
+#include "qom/object.h"
+
+#define TYPE_SYSBUS_MEMORY "sysbus-memory"
+OBJECT_DECLARE_SIMPLE_TYPE(SysBusMemoryState, SYSBUS_MEMORY)
+
+struct SysBusMemoryState {
+    /* <private> */
+    SysBusDevice parent_obj;
+    uint64_t size;
+    bool readonly;
+
+    /* <public> */
+    MemoryRegion mem;
+};
+
+#endif /* HW_SYSBUS_MEMORY_H */
diff --git a/hw/mem/sysbus-memory.c b/hw/mem/sysbus-memory.c
new file mode 100644
index 0000000000..897fa154f0
--- /dev/null
+++ b/hw/mem/sysbus-memory.c
@@ -0,0 +1,83 @@
+/*
+ * QEMU memory SysBusDevice
+ *
+ * Copyright (c) 2021 Greensocs
+ *
+ * Author:
+ * + Damien Hedde
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "hw/mem/sysbus-memory.h"
+#include "hw/qdev-properties.h"
+#include "qemu/log.h"
+#include "qemu/module.h"
+#include "qapi/error.h"
+
+static Property sysbus_memory_properties[] = {
+    DEFINE_PROP_UINT64("size", SysBusMemoryState, size, 0),
+    DEFINE_PROP_BOOL("readonly", SysBusMemoryState, readonly, false),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
+static void sysbus_memory_realize(DeviceState *dev, Error **errp)
+{
+    SysBusMemoryState *s = SYSBUS_MEMORY(dev);
+    gchar *name;
+
+    if (!s->size) {
+        error_setg(errp, "'size' must be non-zero.");
+        return;
+    }
+
+    /*
+     * We impose having an id (which is unique) because we need to generate
+     * a unique name for the memory region.
+     * memory_region_init_ram/rom() will abort() (in qemu_ram_set_idstr()
+     * function if 2 system-memory devices are created with the same name
+     * for the memory region).
+     */
+    if (!dev->id) {
+        error_setg(errp, "system-memory device must have an id.");
+        return;
+    }
+    name = g_strdup_printf("%s.region", dev->id);
+
+    if (s->readonly) {
+        memory_region_init_rom(&s->mem, OBJECT(dev), name, s->size, errp);
+    } else {
+        memory_region_init_ram(&s->mem, OBJECT(dev), name, s->size, errp);
+    }
+
+    g_free(name);
+
+    if (!*errp) {
+        sysbus_init_mmio(SYS_BUS_DEVICE(s), &s->mem);
+    }
+}
+
+static void sysbus_memory_class_init(ObjectClass *klass, void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+
+    dc->user_creatable = true;
+    dc->realize = sysbus_memory_realize;
+    device_class_set_props(dc, sysbus_memory_properties);
+}
+
+static const TypeInfo sysbus_memory_info = {
+    .name          = TYPE_SYSBUS_MEMORY,
+    .parent        = TYPE_SYS_BUS_DEVICE,
+    .instance_size = sizeof(SysBusMemoryState),
+    .class_init    = sysbus_memory_class_init,
+};
+
+static void sysbus_memory_register_types(void)
+{
+    type_register_static(&sysbus_memory_info);
+}
+
+type_init(sysbus_memory_register_types)
diff --git a/hw/mem/meson.build b/hw/mem/meson.build
index 3c8fdef9f9..81e2de1d34 100644
--- a/hw/mem/meson.build
+++ b/hw/mem/meson.build
@@ -7,3 +7,5 @@ mem_ss.add(when: 'CONFIG_NVDIMM', if_true: files('nvdimm.c'))
 softmmu_ss.add_all(when: 'CONFIG_MEM_DEVICE', if_true: mem_ss)
 
 softmmu_ss.add(when: 'CONFIG_FUZZ', if_true: files('sparse-mem.c'))
+
+softmmu_ss.add(files('sysbus-memory.c'))
-- 
2.33.0