memory-device: Support empty memory devices

Let's support empty memory devices -- memory devices that don't have a
memory device region in the current configuration. hv-balloon with an
optional memdev is the primary use case.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>

hw/mem/memory-device.c

@@ -20,6 +20,22 @@
 #include "exec/address-spaces.h"
 #include "trace.h"
 
+static bool memory_device_is_empty(const MemoryDeviceState *md)
+{
+    const MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(md);
+    Error *local_err = NULL;
+    MemoryRegion *mr;
+
+    /* dropping const here is fine as we don't touch the memory region */
+    mr = mdc->get_memory_region((MemoryDeviceState *)md, &local_err);
+    if (local_err) {
+        /* Not empty, we'll report errors later when ontaining the MR again. */
+        error_free(local_err);
+        return false;
+    }
+    return !mr;
+}
+
 static gint memory_device_addr_sort(gconstpointer a, gconstpointer b)
 {
     const MemoryDeviceState *md_a = MEMORY_DEVICE(a);
@@ -249,6 +265,10 @@ static uint64_t memory_device_get_free_addr(MachineState *ms,
         uint64_t next_addr;
         Range tmp;
 
+        if (memory_device_is_empty(md)) {
+            continue;
+        }
+
         range_init_nofail(&tmp, mdc->get_addr(md),
                           memory_device_get_region_size(md, &error_abort));
 
@@ -292,6 +312,7 @@ MemoryDeviceInfoList *qmp_memory_device_list(void)
         const MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(item->data);
         MemoryDeviceInfo *info = g_new0(MemoryDeviceInfo, 1);
 
+        /* Let's query infotmation even for empty memory devices. */
         mdc->fill_device_info(md, info);
 
         QAPI_LIST_APPEND(tail, info);
@@ -311,7 +332,7 @@ static int memory_device_plugged_size(Object *obj, void *opaque)
         const MemoryDeviceState *md = MEMORY_DEVICE(obj);
         const MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(obj);
 
-        if (dev->realized) {
+        if (dev->realized && !memory_device_is_empty(md)) {
             *size += mdc->get_plugged_size(md, &error_abort);
         }
     }
@@ -337,6 +358,11 @@ void memory_device_pre_plug(MemoryDeviceState *md, MachineState *ms,
     uint64_t addr, align = 0;
     MemoryRegion *mr;
 
+    /* We support empty memory devices even without device memory. */
+    if (memory_device_is_empty(md)) {
+        return;
+    }
+
     if (!ms->device_memory) {
         error_setg(errp, "the configuration is not prepared for memory devices"
                          " (e.g., for memory hotplug), consider specifying the"
@@ -380,10 +406,17 @@ out:
 void memory_device_plug(MemoryDeviceState *md, MachineState *ms)
 {
     const MemoryDeviceClass *mdc = MEMORY_DEVICE_GET_CLASS(md);
-    const unsigned int memslots = memory_device_get_memslots(md);
+    unsigned int memslots;
-    const uint64_t addr = mdc->get_addr(md);
+    uint64_t addr;
     MemoryRegion *mr;
 
+    if (memory_device_is_empty(md)) {
+        return;
+    }
+
+    memslots = memory_device_get_memslots(md);
+    addr = mdc->get_addr(md);
+
     /*
      * We expect that a previous call to memory_device_pre_plug() succeeded, so
      * it can't fail at this point.
@@ -408,6 +441,10 @@ void memory_device_unplug(MemoryDeviceState *md, MachineState *ms)
     const unsigned int memslots = memory_device_get_memslots(md);
     MemoryRegion *mr;
 
+    if (memory_device_is_empty(md)) {
+        return;
+    }
+
     /*
      * We expect that a previous call to memory_device_pre_plug() succeeded, so
      * it can't fail at this point.

include/hw/mem/memory-device.h

@@ -38,6 +38,10 @@ typedef struct MemoryDeviceState MemoryDeviceState;
  * address in guest physical memory can either be specified explicitly
  * or get assigned automatically.
  *
+ * Some memory device might not own a memory region in certain device
+ * configurations. Such devices can logically get (un)plugged, however,
+ * empty memory devices are mostly ignored by the memory device code.
+ *
  * Conceptually, memory devices only span one memory region. If multiple
  * successive memory regions are used, a covering memory region has to
  * be provided. Scattered memory regions are not supported for single
@@ -91,7 +95,8 @@ struct MemoryDeviceClass {
     uint64_t (*get_plugged_size)(const MemoryDeviceState *md, Error **errp);
 
     /*
-     * Return the memory region of the memory device.
+     * Return the memory region of the memory device. If the device is
+     * completely empty, returns NULL without an error.
      *
      * Called when (un)plugging the memory device, to (un)map the
      * memory region in guest physical memory, but also to detect the