virtio-mem: add memory via add_memory_driver_managed()

Virtio-mem managed memory is always detected and added by the virtio-mem
driver, never using something like the firmware-provided memory map.
This is the case after an ordinary system reboot, and has to be guaranteed
after kexec. Especially, virtio-mem added memory resources can contain
inaccessible parts ("unblocked memory blocks"), blindly forwarding them
to a kexec kernel is dangerous, as unplugged memory will get accessed
(esp. written).

Let's use the new way of adding special driver-managed memory introduced
in commit 7b7b2721 ("mm/memory_hotplug: introduce
add_memory_driver_managed()").

This will result in no entries in /sys/firmware/memmap ("raw firmware-
provided memory map"), the memory resource will be flagged
IORESOURCE_MEM_DRIVER_MANAGED (esp., kexec_file_load() will not place
kexec images on this memory), and it is exposed as "System RAM
(virtio_mem)" in /proc/iomem, so esp. kexec-tools can properly handle it.

Example /proc/iomem before this change:
  [...]
  140000000-333ffffff : virtio0
    140000000-147ffffff : System RAM
  334000000-533ffffff : virtio1
    338000000-33fffffff : System RAM
    340000000-347ffffff : System RAM
    348000000-34fffffff : System RAM
  [...]

Example /proc/iomem after this change:
  [...]
  140000000-333ffffff : virtio0
    140000000-147ffffff : System RAM (virtio_mem)
  334000000-533ffffff : virtio1
    338000000-33fffffff : System RAM (virtio_mem)
    340000000-347ffffff : System RAM (virtio_mem)
    348000000-34fffffff : System RAM (virtio_mem)
  [...]

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: teawater <teawaterz@linux.alibaba.com>
Fixes: 5f1f79bb ("virtio-mem: Paravirtualized memory hotplug")
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200611093518.5737-1-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>

drivers/virtio/virtio_mem.c

@@ -101,6 +101,11 @@ struct virtio_mem {
 
 	/* The parent resource for all memory added via this device. */
 	struct resource *parent_resource;
+	/*
+	 * Copy of "System RAM (virtio_mem)" to be used for
+	 * add_memory_driver_managed().
+	 */
+	const char *resource_name;
 
 	/* Summary of all memory block states. */
 	unsigned long nb_mb_state[VIRTIO_MEM_MB_STATE_COUNT];
@@ -414,8 +419,20 @@ static int virtio_mem_mb_add(struct virtio_mem *vm, unsigned long mb_id)
 	if (nid == NUMA_NO_NODE)
 		nid = memory_add_physaddr_to_nid(addr);
 
+	/*
+	 * When force-unloading the driver and we still have memory added to
+	 * Linux, the resource name has to stay.
+	 */
+	if (!vm->resource_name) {
+		vm->resource_name = kstrdup_const("System RAM (virtio_mem)",
+						  GFP_KERNEL);
+		if (!vm->resource_name)
+			return -ENOMEM;
+	}
+
 	dev_dbg(&vm->vdev->dev, "adding memory block: %lu\n", mb_id);
-	return add_memory(nid, addr, memory_block_size_bytes());
+	return add_memory_driver_managed(nid, addr, memory_block_size_bytes(),
+					 vm->resource_name);
 }
 
 /*
@@ -1890,10 +1907,12 @@ static void virtio_mem_remove(struct virtio_device *vdev)
 	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL] ||
 	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE] ||
 	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL] ||
-	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE])
+	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE]) {
 		dev_warn(&vdev->dev, "device still has system memory added\n");
-	else
+	} else {
 		virtio_mem_delete_resource(vm);
+		kfree_const(vm->resource_name);
+	}
 
 	/* remove all tracking data - no locking needed */
 	vfree(vm->mb_state);