pc-dimm: factor out MemoryDevice

On the qmp level, we already have the concept of memory devices:
"query-memory-devices"
Right now, we only support NVDIMM and PCDIMM.

We want to map other devices later into the address space of the guest. Such device could e.g. be virtio devices. These devices will have a guest memory range assigned but won't be exposed via e.g. ACPI. We want to make them look like memory device, but not glued to pc-dimm.

Especially, it will not always be possible to have TYPE_PC_DIMM as a parent class (e.g. virtio devices). Let's use an interface instead. As a first part, convert handling of
- qmp_pc_dimm_device_list
- get_plugged_memory_size
to our new model. plug/unplug stuff etc. will follow later.

A memory device will have to provide the following functions:
- get_addr(): Necessary, as the property "addr" can e.g. not be used for virtio devices (already defined).

- get_plugged_size(): The amount this device offers to the guest as of now.
- get_region_size(): Because this can later on be bigger than the plugged size.
- fill_device_info(): Fill MemoryDeviceInfo, e.g. for qmp.

Signed-off-by David Hildenbrand <[email protected]>

---
 hw/i386/acpi-build.c                         |   3 +-
 hw/mem/Makefile.objs                         |   1 +
 hw/mem/memory-device.c                       | 119 +++++++++++++++++++++++++++
 hw/mem/pc-dimm.c                             | 119 ++++++++++++++-------------
 hw/ppc/spapr.c                               |   3 +-
 hw/ppc/spapr_hcall.c                         |   1 +
 include/hw/mem/memory-device.h               |  44 ++++++++++
 include/hw/mem/pc-dimm.h                     |   2 -
 numa.c                                       |   3 +-
 qmp.c                                        |   4 +-
 stubs/Makefile.objs                          |   2 +-
 stubs/{qmp_pc_dimm.c => qmp_memory_device.c} |   4 +-
 12 files changed, 239 insertions(+), 66 deletions(-)
 create mode 100644 hw/mem/memory-device.c
 create mode 100644 include/hw/mem/memory-device.h
 rename stubs/{qmp_pc_dimm.c => qmp_memory_device.c} (61%)

hw/i386/acpi-build.c +2 -1

index 3cf2a1679c..ca3645d57b 100644
@@ -46,6 +46,7 @@
 #include "hw/acpi/vmgenid.h"
 #include "sysemu/tpm_backend.h"
 #include "hw/timer/mc146818rtc_regs.h"
+#include "hw/mem/memory-device.h"
 #include "sysemu/numa.h"
 
 /* Supported chipsets: */

@@ -2253,7 +2254,7 @@ build_tpm2(GArray *table_data, BIOSLinker *linker, GArray *tcpalog)
 static void build_srat_hotpluggable_memory(GArray *table_data, uint64_t base,
                                            uint64_t len, int default_node)
 {
-    MemoryDeviceInfoList *info_list = qmp_pc_dimm_device_list();
+    MemoryDeviceInfoList *info_list = qmp_memory_device_list();
     MemoryDeviceInfoList *info;
     MemoryDeviceInfo *mi;
     PCDIMMDeviceInfo *di;

hw/mem/Makefile.objs +1 -0

index f12f8b97a2..10be4df2a2 100644
@@ -1,2 +1,3 @@
 common-obj-$(CONFIG_MEM_HOTPLUG) += pc-dimm.o
+common-obj-$(CONFIG_MEM_HOTPLUG) += memory-device.o
 common-obj-$(CONFIG_NVDIMM) += nvdimm.o

hw/mem/memory-device.c new file +13 -0

new file mode 100644
index 0000000000..0e0d6b539a
@@ -0,0 +1,119 @@
+/*
+ * Memory Device Interface
+ *
+ * Copyright ProfitBricks GmbH 2012
+ * Copyright (C) 2014 Red Hat Inc
+ * Copyright (c) 2018 Red Hat Inc
+ *
+ * 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/memory-device.h"

hw/mem/memory-device.c new file +96 -0

+    const uint64_t addr_a = mdc->get_addr(md_a);
+    const uint64_t addr_b = mdc->get_addr(md_b);
+
+    if (addr_a > addr_b) {
+        return 1;
+    } else if (addr_a < addr_b) {
+        return -1;
+    }
+    return 0;
+}
+
+static int memory_device_built_list(Object *obj, void *opaque)
+{
+    GSList **list = opaque;
+
+    if (object_dynamic_cast(obj, TYPE_MEMORY_DEVICE)) {
+        DeviceState *dev = DEVICE(obj);
+        if (dev->realized) { /* only realized memory devices matter */
+            *list = g_slist_insert_sorted(*list, dev, memory_device_addr_sort);
+        }
+    }
+
+    object_child_foreach(obj, memory_device_built_list, opaque);
+    return 0;
+}
+
+MemoryDeviceInfoList *qmp_memory_device_list(void)
+{
+    GSList *devices = NULL, *item;
+    MemoryDeviceInfoList *list = NULL, *prev = NULL;
+
+    object_child_foreach(qdev_get_machine(), memory_device_built_list,
+                         &devices);
+
+    for (item = devices; item; item = g_slist_next(item)) {
+        MemoryDeviceState *md = MEMORY_DEVICE(item->data);
+        MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(item->data);
+        MemoryDeviceInfoList *elem = g_new0(MemoryDeviceInfoList, 1);
+        MemoryDeviceInfo *info = g_new0(MemoryDeviceInfo, 1);
+
+        mdc->fill_device_info(md, info);
+
+        elem->value = info;
+        elem->next = NULL;
+        if (prev) {
+            prev->next = elem;
+        } else {
+            list = elem;
+        }
+        prev = elem;
+    }
+
+    g_slist_free(devices);
+
+    return list;
+}
+
+static int memory_device_plugged_size(Object *obj, void *opaque)
+{
+    uint64_t *size = opaque;
+
+    if (object_dynamic_cast(obj, TYPE_MEMORY_DEVICE)) {
+        DeviceState *dev = DEVICE(obj);
+        MemoryDeviceState *md = MEMORY_DEVICE(obj);
+        MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(obj);
+
+        if (dev->realized) {
+            *size += mdc->get_plugged_size(md, &error_abort);
+        }
+    }
+
+    object_child_foreach(obj, memory_device_plugged_size, opaque);
+    return 0;
+}
+
+uint64_t get_plugged_memory_size(void)
+{
+    uint64_t size = 0;
+
+    memory_device_plugged_size(qdev_get_machine(), &size);
+
+    return size;
+}
+
+static const TypeInfo memory_device_info = {
+    .name          = TYPE_MEMORY_DEVICE,
+    .parent        = TYPE_INTERFACE,
+    .class_size = sizeof(MemoryDeviceClass),
+};
+
+static void memory_device_register_types(void)
+{
+    type_register_static(&memory_device_info);
+}
+
+type_init(memory_device_register_types)

hw/mem/pc-dimm.c +63 -56

index 51350d9c2d..1dbf699e02 100644
@@ -21,6 +21,7 @@
 #include "qemu/osdep.h"
 #include "hw/mem/pc-dimm.h"
 #include "hw/mem/nvdimm.h"
+#include "hw/mem/memory-device.h"
 #include "qapi/error.h"
 #include "qemu/config-file.h"
 #include "qapi/visitor.h"

@@ -158,11 +159,6 @@ uint64_t pc_existing_dimms_capacity(Error **errp)
     return cap.size;
 }
 
-uint64_t get_plugged_memory_size(void)
-{
-    return pc_existing_dimms_capacity(&error_abort);
-}
-
 static int pc_dimm_slot2bitmap(Object *obj, void *opaque)
 {
     unsigned long *bitmap = opaque;

@@ -238,57 +234,6 @@ static int pc_dimm_built_list(Object *obj, void *opaque)
     return 0;
 }
 
-MemoryDeviceInfoList *qmp_pc_dimm_device_list(void)
-{
-    GSList *dimms = NULL, *item;
-    MemoryDeviceInfoList *list = NULL, *prev = NULL;
-
-    object_child_foreach(qdev_get_machine(), pc_dimm_built_list, &dimms);
-
-    for (item = dimms; item; item = g_slist_next(item)) {
-        PCDIMMDevice *dimm = PC_DIMM(item->data);
-        Object *obj = OBJECT(dimm);
-        MemoryDeviceInfoList *elem = g_new0(MemoryDeviceInfoList, 1);
-        MemoryDeviceInfo *info = g_new0(MemoryDeviceInfo, 1);
-        PCDIMMDeviceInfo *di = g_new0(PCDIMMDeviceInfo, 1);
-        bool is_nvdimm = object_dynamic_cast(obj, TYPE_NVDIMM);
-        DeviceClass *dc = DEVICE_GET_CLASS(obj);
-        DeviceState *dev = DEVICE(obj);
-
-        if (dev->id) {
-            di->has_id = true;
-            di->id = g_strdup(dev->id);
-        }
-        di->hotplugged = dev->hotplugged;
-        di->hotpluggable = dc->hotpluggable;
-        di->addr = dimm->addr;
-        di->slot = dimm->slot;
-        di->node = dimm->node;
-        di->size = object_property_get_uint(obj, PC_DIMM_SIZE_PROP, NULL);
-        di->memdev = object_get_canonical_path(OBJECT(dimm->hostmem));
-
-        if (!is_nvdimm) {
-            info->u.dimm.data = di;
-            info->type = MEMORY_DEVICE_INFO_KIND_DIMM;
-        } else {
-            info->u.nvdimm.data = di;
-            info->type = MEMORY_DEVICE_INFO_KIND_NVDIMM;
-        }
-        elem->value = info;
-        elem->next = NULL;
-        if (prev) {
-            prev->next = elem;
-        } else {
-            list = elem;
-        }
-        prev = elem;
-    }
-
-    g_slist_free(dimms);
-
-    return list;
-}
-
 uint64_t pc_dimm_get_free_addr(uint64_t address_space_start,
                                uint64_t address_space_size,
                                uint64_t *hint, uint64_t align, uint64_t size,

@@ -445,10 +390,62 @@ static MemoryRegion *pc_dimm_get_vmstate_memory_region(PCDIMMDevice *dimm)
     return host_memory_backend_get_memory(dimm->hostmem, &error_abort);
 }
 
+static uint64_t pc_dimm_md_get_addr(MemoryDeviceState *md)
+{
+    PCDIMMDevice *dimm = PC_DIMM(md);
+
+    return dimm->addr;
+}
+
+static uint64_t pc_dimm_md_get_region_size(MemoryDeviceState *md, Error **errp)
+{
+    PCDIMMDevice *dimm = PC_DIMM(md);
+    PCDIMMDeviceClass *ddc = PC_DIMM_GET_CLASS(md);
+    MemoryRegion *mr;
+
+    mr = ddc->get_memory_region(dimm, errp);
+    if (!mr) {
+        return 0;
+    }
+
+    return memory_region_size(mr);
+}
+
+static void pc_dimm_md_fill_device_info(MemoryDeviceState *md,
+                                        MemoryDeviceInfo *info)
+{
+    PCDIMMDeviceInfo *di = g_new0(PCDIMMDeviceInfo, 1);
+    DeviceClass *dc = DEVICE_GET_CLASS(md);
+    PCDIMMDevice *dimm = PC_DIMM(md);
+    DeviceState *dev = DEVICE(md);
+
+    if (dev->id) {
+        di->has_id = true;
+        di->id = g_strdup(dev->id);
+    }
+    di->hotplugged = dev->hotplugged;
+    di->hotpluggable = dc->hotpluggable;
+    di->addr = dimm->addr;
+    di->slot = dimm->slot;
+    di->node = dimm->node;
+    di->size = object_property_get_uint(OBJECT(dimm), PC_DIMM_SIZE_PROP,
+                                        NULL);
+    di->memdev = object_get_canonical_path(OBJECT(dimm->hostmem));
+
+    if (object_dynamic_cast(OBJECT(dev), TYPE_NVDIMM)) {
+        info->u.nvdimm.data = di;
+        info->type = MEMORY_DEVICE_INFO_KIND_NVDIMM;
+    } else {
+        info->u.dimm.data = di;
+        info->type = MEMORY_DEVICE_INFO_KIND_DIMM;
+    }
+}
+
 static void pc_dimm_class_init(ObjectClass *oc, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(oc);
     PCDIMMDeviceClass *ddc = PC_DIMM_CLASS(oc);
+    MemoryDeviceClass *mdc = MEMORY_DEVICE_CLASS(oc);
 
     dc->realize = pc_dimm_realize;
     dc->unrealize = pc_dimm_unrealize;

@@ -457,6 +454,12 @@ static void pc_dimm_class_init(ObjectClass *oc, void *data)
 
     ddc->get_memory_region = pc_dimm_get_memory_region;
     ddc->get_vmstate_memory_region = pc_dimm_get_vmstate_memory_region;
+
+    mdc->get_addr = pc_dimm_md_get_addr;
+    /* for a dimm plugged_size == region_size */
+    mdc->get_plugged_size = pc_dimm_md_get_region_size;
+    mdc->get_region_size = pc_dimm_md_get_region_size;
+    mdc->fill_device_info = pc_dimm_md_fill_device_info;
 }
 
 static TypeInfo pc_dimm_info = {

@@ -466,6 +469,10 @@ static TypeInfo pc_dimm_info = {
     .instance_init = pc_dimm_init,
     .class_init    = pc_dimm_class_init,
     .class_size    = sizeof(PCDIMMDeviceClass),
+    .interfaces = (InterfaceInfo[]) {
+        { TYPE_MEMORY_DEVICE },
+        { }
+    },
 };
 
 static void pc_dimm_register_types(void)

hw/ppc/spapr.c +2 -1

index a81570e7c8..a7428f7da7 100644
@@ -74,6 +74,7 @@
 #include "hw/compat.h"
 #include "qemu/cutils.h"
 #include "hw/ppc/spapr_cpu_core.h"
+#include "hw/mem/memory-device.h"
 
 #include <libfdt.h>
 

@@ -722,7 +723,7 @@ static int spapr_populate_drconf_memory(sPAPRMachineState *spapr, void *fdt)
     }
 
     if (hotplug_lmb_start) {
-        dimms = qmp_pc_dimm_device_list();
+        dimms = qmp_memory_device_list();
     }
 
     /* ibm,dynamic-memory */

hw/ppc/spapr_hcall.c +1 -0

index 16bccdd5c0..4cdae3ca3a 100644
@@ -14,6 +14,7 @@
 #include "kvm_ppc.h"
 #include "hw/ppc/spapr_ovec.h"
 #include "mmu-book3s-v3.h"
+#include "hw/mem/memory-device.h"
 
 struct SPRSyncState {
     int spr;

include/hw/mem/memory-device.h new file +44 -0

new file mode 100644
index 0000000000..3e498b2e61
@@ -0,0 +1,44 @@
+/*
+ * Memory Device Interface
+ *
+ * Copyright (c) 2018 Red Hat, Inc.
+ *
+ * Authors:
+ *  David Hildenbrand <[email protected]>
+ *
+ * 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 MEMORY_DEVICE_H
+#define MEMORY_DEVICE_H
+
+#include "qom/object.h"
+#include "hw/qdev.h"
+
+#define TYPE_MEMORY_DEVICE "memory-device"
+
+#define MEMORY_DEVICE_CLASS(klass) \
+     OBJECT_CLASS_CHECK(MemoryDeviceClass, (klass), TYPE_MEMORY_DEVICE)
+#define MEMORY_DEVICE_GET_CLASS(obj) \
+    OBJECT_GET_CLASS(MemoryDeviceClass, (obj), TYPE_MEMORY_DEVICE)
+#define MEMORY_DEVICE(obj) \
+     INTERFACE_CHECK(MemoryDeviceState, (obj), TYPE_MEMORY_DEVICE)
+
+typedef struct MemoryDeviceState {
+    Object parent_obj;
+} MemoryDeviceState;
+
+typedef struct MemoryDeviceClass {
+    InterfaceClass parent_class;
+
+    uint64_t (*get_addr)(MemoryDeviceState *md);
+    uint64_t (*get_plugged_size)(MemoryDeviceState *md, Error **errp);
+    uint64_t (*get_region_size)(MemoryDeviceState *md, Error **errp);
+    void (*fill_device_info)(MemoryDeviceState *md, MemoryDeviceInfo *info);
+} MemoryDeviceClass;
+
+MemoryDeviceInfoList *qmp_memory_device_list(void);
+uint64_t get_plugged_memory_size(void);
+
+#endif

include/hw/mem/pc-dimm.h +0 -2

index 1fc479281c..e88073321f 100644
@@ -93,9 +93,7 @@ uint64_t pc_dimm_get_free_addr(uint64_t address_space_start,
 
 int pc_dimm_get_free_slot(const int *hint, int max_slots, Error **errp);
 
-MemoryDeviceInfoList *qmp_pc_dimm_device_list(void);
 uint64_t pc_existing_dimms_capacity(Error **errp);
-uint64_t get_plugged_memory_size(void);
 void pc_dimm_memory_plug(DeviceState *dev, MemoryHotplugState *hpms,
                          MemoryRegion *mr, uint64_t align, Error **errp);
 void pc_dimm_memory_unplug(DeviceState *dev, MemoryHotplugState *hpms,

numa.c +2 -1

index 1116c90af9..6a0eaebc01 100644
@@ -36,6 +36,7 @@
 #include "hw/boards.h"
 #include "sysemu/hostmem.h"
 #include "hw/mem/pc-dimm.h"
+#include "hw/mem/memory-device.h"
 #include "qemu/option.h"
 #include "qemu/config-file.h"
 #include "qemu/cutils.h"

@@ -520,7 +521,7 @@ void memory_region_allocate_system_memory(MemoryRegion *mr, Object *owner,
 
 static void numa_stat_memory_devices(NumaNodeMem node_mem[])
 {
-    MemoryDeviceInfoList *info_list = qmp_pc_dimm_device_list();
+    MemoryDeviceInfoList *info_list = qmp_memory_device_list();
     MemoryDeviceInfoList *info;
     PCDIMMDeviceInfo     *pcdimm_info;
 

qmp.c +2 -2

index f72261667f..3de029946a 100644
@@ -39,7 +39,7 @@
 #include "qapi/qobject-input-visitor.h"
 #include "hw/boards.h"
 #include "qom/object_interfaces.h"
-#include "hw/mem/pc-dimm.h"
+#include "hw/mem/memory-device.h"
 #include "hw/acpi/acpi_dev_interface.h"
 
 NameInfo *qmp_query_name(Error **errp)

@@ -731,7 +731,7 @@ void qmp_object_del(const char *id, Error **errp)
 
 MemoryDeviceInfoList *qmp_query_memory_devices(Error **errp)
 {
-    return qmp_pc_dimm_device_list();
+    return qmp_memory_device_list();
 }
 
 ACPIOSTInfoList *qmp_query_acpi_ospm_status(Error **errp)

stubs/Makefile.objs +1 -1

index 2d59d84091..53d3f32cb2 100644
@@ -34,7 +34,7 @@ stub-obj-y += uuid.o
 stub-obj-y += vm-stop.o
 stub-obj-y += vmstate.o
 stub-obj-$(CONFIG_WIN32) += fd-register.o
-stub-obj-y += qmp_pc_dimm.o
+stub-obj-y += qmp_memory_device.o
 stub-obj-y += target-monitor-defs.o
 stub-obj-y += target-get-monitor-def.o
 stub-obj-y += pc_madt_cpu_entry.o

stubs/qmp_memory_device.c renamed from stubs/qmp_pc_dimm.c +2 -2

similarity index 61%
rename from stubs/qmp_pc_dimm.c
rename to stubs/qmp_memory_device.c
index b6b2cca89e..85ff8f2d7e 100644
@@ -1,8 +1,8 @@
 #include "qemu/osdep.h"
 #include "qom/object.h"
-#include "hw/mem/pc-dimm.h"
+#include "hw/mem/memory-device.h"
 
-MemoryDeviceInfoList *qmp_pc_dimm_device_list(void)
+MemoryDeviceInfoList *qmp_memory_device_list(void)
 {
    return NULL;
 }

Quoted 16 lines · David Hildenbrand

On the qmp level, we already have the concept of memory devices:
"query-memory-devices"
Right now, we only support NVDIMM and PCDIMM.

We want to map other devices later into the address space of the guest. Such device could e.g. be virtio devices. These devices will have a guest memory range assigned but won't be exposed via e.g. ACPI. We want to make them look like memory device, but not glued to pc-dimm.

Especially, it will not always be possible to have TYPE_PC_DIMM as a parent class (e.g. virtio devices). Let's use an interface instead. As a first part, convert handling of
- qmp_pc_dimm_device_list
- get_plugged_memory_size
to our new model. plug/unplug stuff etc. will follow later.

Quoted 3 lines

A memory device will have to provide the following functions:
- get_addr(): Necessary, as the property "addr" can e.g. not be used for virtio devices (already defined).

Is the assumption here that a memory device has a fixed RAM address (GPA) from when it is realized? It seems to me plausible that you could have a memory device that has a specific size, but has a BAR of some sort that's programmed by the guest as part of the "plug" process.

Quoted 81 lines

- get_plugged_size(): The amount this device offers to the guest as of now.
- get_region_size(): Because this can later on be bigger than the plugged size.
- fill_device_info(): Fill MemoryDeviceInfo, e.g. for qmp.

Signed-off-by David Hildenbrand <[email protected]>

---
 hw/i386/acpi-build.c                         |   3 +-
 hw/mem/Makefile.objs                         |   1 +
 hw/mem/memory-device.c                       | 119 +++++++++++++++++++++++++++
 hw/mem/pc-dimm.c                             | 119 ++++++++++++++-------------
 hw/ppc/spapr.c                               |   3 +-
 hw/ppc/spapr_hcall.c                         |   1 +
 include/hw/mem/memory-device.h               |  44 ++++++++++
 include/hw/mem/pc-dimm.h                     |   2 -
 numa.c                                       |   3 +-
 qmp.c                                        |   4 +-
 stubs/Makefile.objs                          |   2 +-
 stubs/{qmp_pc_dimm.c => qmp_memory_device.c} |   4 +-
 12 files changed, 239 insertions(+), 66 deletions(-)
 create mode 100644 hw/mem/memory-device.c
 create mode 100644 include/hw/mem/memory-device.h
 rename stubs/{qmp_pc_dimm.c => qmp_memory_device.c} (61%)

hw/i386/acpi-build.c +2 -1

index 3cf2a1679c..ca3645d57b 100644
@@ -46,6 +46,7 @@
 #include "hw/acpi/vmgenid.h"
 #include "sysemu/tpm_backend.h"
 #include "hw/timer/mc146818rtc_regs.h"
+#include "hw/mem/memory-device.h"
 #include "sysemu/numa.h"
 
 /* Supported chipsets: */

@@ -2253,7 +2254,7 @@ build_tpm2(GArray *table_data, BIOSLinker *linker, GArray *tcpalog)
 static void build_srat_hotpluggable_memory(GArray *table_data, uint64_t base,
                                            uint64_t len, int default_node)
 {
-    MemoryDeviceInfoList *info_list = qmp_pc_dimm_device_list();
+    MemoryDeviceInfoList *info_list = qmp_memory_device_list();
     MemoryDeviceInfoList *info;
     MemoryDeviceInfo *mi;
     PCDIMMDeviceInfo *di;

hw/mem/Makefile.objs +1 -0

index f12f8b97a2..10be4df2a2 100644
@@ -1,2 +1,3 @@
 common-obj-$(CONFIG_MEM_HOTPLUG) += pc-dimm.o
+common-obj-$(CONFIG_MEM_HOTPLUG) += memory-device.o
 common-obj-$(CONFIG_NVDIMM) += nvdimm.o

hw/mem/memory-device.c new file +20 -0

new file mode 100644
index 0000000000..0e0d6b539a
@@ -0,0 +1,119 @@
+/*
+ * Memory Device Interface
+ *
+ * Copyright ProfitBricks GmbH 2012
+ * Copyright (C) 2014 Red Hat Inc
+ * Copyright (c) 2018 Red Hat Inc
+ *
+ * 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/memory-device.h"
+#include "hw/qdev.h"
+#include "qapi/error.h"
+#include "hw/boards.h"
+#include "qemu/range.h"
+
+static gint memory_device_addr_sort(gconstpointer a, gconstpointer b)
+{

hw/mem/memory-device.c line 23

Quoted 3 lines

hw/mem/memory-device.c new file +3 -0

+    MemoryDeviceState *md_a = MEMORY_DEVICE(a);
+    MemoryDeviceState *md_b = MEMORY_DEVICE(b);
+    MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(a);

AFAICT from the code below, the two devices don't necessarily have to have the same class, so I think you need to get the class pointers for each of them separately. (Or if they do have to be the same by design, I think there should at least be an assert).

Quoted 11 lines

hw/mem/memory-device.c new file +10 -0

+    const uint64_t addr_a = mdc->get_addr(md_a);
+    const uint64_t addr_b = mdc->get_addr(md_b);
+
+    if (addr_a > addr_b) {
+        return 1;
+    } else if (addr_a < addr_b) {
+        return -1;
+    }
+    return 0;
+}