[PATCH v2 2/6] memory: Change memory_region_set_ram_discard_manager() to return the result

[Chenyi Qiang <chenyi.qiang@xxxxxxxxx>]

Modify memory_region_set_ram_discard_manager() to return false if a
RamDiscardManager is already set in the MemoryRegion. The caller must
handle this failure, such as having virtio-mem undo its actions and fail
the realize() process. Opportunistically move the call earlier to avoid
complex error handling.

This change is beneficial when introducing a new RamDiscardManager
instance besides virtio-mem. After
ram_block_coordinated_discard_require(true) unlocks all
RamDiscardManager instances, only one instance is allowed to be set for
a MemoryRegion at present.

Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
---
Changes in v2:
    - newly added.
---
 hw/virtio/virtio-mem.c | 30 +++++++++++++++++-------------
 include/exec/memory.h  |  6 +++---
 system/memory.c        | 11 ++++++++---
 3 files changed, 28 insertions(+), 19 deletions(-)

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index 21f16e4912..ef818a2cdf 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -1074,6 +1074,18 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
                         vmem->block_size;
     vmem->bitmap = bitmap_new(vmem->bitmap_size);
 
+    /*
+     * Set ourselves as RamDiscardManager before the plug handler maps the
+     * memory region and exposes it via an address space.
+     */
+    if (memory_region_set_ram_discard_manager(&vmem->memdev->mr,
+                                              RAM_DISCARD_MANAGER(vmem))) {
+        error_setg(errp, "Failed to set RamDiscardManager");
+        g_free(vmem->bitmap);
+        ram_block_coordinated_discard_require(false);
+        return;
+    }
+
     virtio_init(vdev, VIRTIO_ID_MEM, sizeof(struct virtio_mem_config));
     vmem->vq = virtio_add_queue(vdev, 128, virtio_mem_handle_request);
 
@@ -1124,13 +1136,6 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
     vmem->system_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
     vmem->system_reset->vmem = vmem;
     qemu_register_resettable(obj);
-
-    /*
-     * Set ourselves as RamDiscardManager before the plug handler maps the
-     * memory region and exposes it via an address space.
-     */
-    memory_region_set_ram_discard_manager(&vmem->memdev->mr,
-                                          RAM_DISCARD_MANAGER(vmem));
 }
 
 static void virtio_mem_device_unrealize(DeviceState *dev)
@@ -1138,12 +1143,6 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VirtIOMEM *vmem = VIRTIO_MEM(dev);
 
-    /*
-     * The unplug handler unmapped the memory region, it cannot be
-     * found via an address space anymore. Unset ourselves.
-     */
-    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
-
     qemu_unregister_resettable(OBJECT(vmem->system_reset));
     object_unref(OBJECT(vmem->system_reset));
 
@@ -1155,6 +1154,11 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
     host_memory_backend_set_mapped(vmem->memdev, false);
     virtio_del_queue(vdev, 0);
     virtio_cleanup(vdev);
+    /*
+     * The unplug handler unmapped the memory region, it cannot be
+     * found via an address space anymore. Unset ourselves.
+     */
+    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
     g_free(vmem->bitmap);
     ram_block_coordinated_discard_require(false);
 }
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 3bebc43d59..390477b588 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -2487,13 +2487,13 @@ static inline bool memory_region_has_ram_discard_manager(MemoryRegion *mr)
  *
  * This function must not be called for a mapped #MemoryRegion, a #MemoryRegion
  * that does not cover RAM, or a #MemoryRegion that already has a
- * #RamDiscardManager assigned.
+ * #RamDiscardManager assigned. Return 0 if the rdm is set successfully.
  *
  * @mr: the #MemoryRegion
  * @rdm: #RamDiscardManager to set
  */
-void memory_region_set_ram_discard_manager(MemoryRegion *mr,
-                                           RamDiscardManager *rdm);
+int memory_region_set_ram_discard_manager(MemoryRegion *mr,
+                                          RamDiscardManager *rdm);
 
 /**
  * memory_region_find: translate an address/size relative to a
diff --git a/system/memory.c b/system/memory.c
index b17b5538ff..297a3dbcd4 100644
--- a/system/memory.c
+++ b/system/memory.c
@@ -2115,12 +2115,17 @@ RamDiscardManager *memory_region_get_ram_discard_manager(MemoryRegion *mr)
     return mr->rdm;
 }
 
-void memory_region_set_ram_discard_manager(MemoryRegion *mr,
-                                           RamDiscardManager *rdm)
+int memory_region_set_ram_discard_manager(MemoryRegion *mr,
+                                          RamDiscardManager *rdm)
 {
     g_assert(memory_region_is_ram(mr));
-    g_assert(!rdm || !mr->rdm);
+    if (mr->rdm && rdm != NULL) {
+        return -1;
+    }
+
+    /* !rdm || !mr->rdm */
     mr->rdm = rdm;
+    return 0;
 }
 
 uint64_t ram_discard_manager_get_min_granularity(const RamDiscardManager *rdm,
-- 
2.43.5