virtio-mem: Prepare for device unplug support

In many cases, blindly unplugging a virtio-mem device is problematic. We
can only safely remove a device once:
* The guest is not expecting to be able to read unplugged memory
  (unplugged-inaccessible == on)
* The virtio-mem device does not have memory plugged (size == 0)
* The virtio-mem device does not have outstanding requests to the VM to
  plug memory (requested-size == 0)

So let's add a callback to the virtio-mem device class to check for that.
We'll wire-up virtio-mem-pci next.

Message-ID: <20230711153445.514112-7-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/virtio/virtio-mem.c

@@ -1512,6 +1512,30 @@ static void virtio_mem_rdm_unregister_listener(RamDiscardManager *rdm,
     QLIST_REMOVE(rdl, next);
 }
 
+static void virtio_mem_unplug_request_check(VirtIOMEM *vmem, Error **errp)
+{
+    if (vmem->unplugged_inaccessible == ON_OFF_AUTO_OFF) {
+        /*
+         * We could allow it with a usable region size of 0, but let's just
+         * not care about that legacy setting.
+         */
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_UNPLUGGED_INACCESSIBLE_PROP "' != 'on'");
+        return;
+    }
+
+    if (vmem->size) {
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_SIZE_PROP "' != '0'");
+        return;
+    }
+    if (vmem->requested_size) {
+        error_setg(errp, "virtio-mem device cannot get unplugged while"
+                   " '" VIRTIO_MEM_REQUESTED_SIZE_PROP "' != '0'");
+        return;
+    }
+}
+
 static void virtio_mem_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(klass);
@@ -1534,6 +1558,7 @@ static void virtio_mem_class_init(ObjectClass *klass, void *data)
     vmc->get_memory_region = virtio_mem_get_memory_region;
     vmc->add_size_change_notifier = virtio_mem_add_size_change_notifier;
     vmc->remove_size_change_notifier = virtio_mem_remove_size_change_notifier;
+    vmc->unplug_request_check = virtio_mem_unplug_request_check;
 
     rdmc->get_min_granularity = virtio_mem_rdm_get_min_granularity;
     rdmc->is_populated = virtio_mem_rdm_is_populated;

include/hw/virtio/virtio-mem.h

@@ -98,6 +98,7 @@ struct VirtIOMEMClass {
     MemoryRegion *(*get_memory_region)(VirtIOMEM *vmem, Error **errp);
     void (*add_size_change_notifier)(VirtIOMEM *vmem, Notifier *notifier);
     void (*remove_size_change_notifier)(VirtIOMEM *vmem, Notifier *notifier);
+    void (*unplug_request_check)(VirtIOMEM *vmem, Error **errp);
 };
 
 #endif