virtio-mem: don't read big block size in Sub Block Mode

We are reading a Big Block Mode value while in Sub Block Mode
when initializing. Fortunately, vm->bbm.bb_size maps to some counter
in the vm->sbm.mb_count array, which is 0 at that point in time.

No harm done; still, this was unintended and is not future-proof.

Fixes: 4ba50cd ("virtio-mem: Big Block Mode (BBM) memory hotplug")
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20210602185720.31821-2-david@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: davidhildenbrand

drivers/virtio/virtio_mem.c

@@ -2420,6 +2420,10 @@ static int virtio_mem_init(struct virtio_mem *vm)
 		dev_warn(&vm->vdev->dev,
 			 "Some device memory is not addressable/pluggable. This can make some memory unusable.\n");
 
+	/* Prepare the offline threshold - make sure we can add two blocks. */
+	vm->offline_threshold = max_t(uint64_t, 2 * memory_block_size_bytes(),
+				      VIRTIO_MEM_DEFAULT_OFFLINE_THRESHOLD);
+
 	/*
 	 * We want subblocks to span at least MAX_ORDER_NR_PAGES and
 	 * pageblock_nr_pages pages. This:
@@ -2466,14 +2470,11 @@ static int virtio_mem_init(struct virtio_mem *vm)
 		       vm->bbm.bb_size - 1;
 		vm->bbm.first_bb_id = virtio_mem_phys_to_bb_id(vm, addr);
 		vm->bbm.next_bb_id = vm->bbm.first_bb_id;
-	}
 
-	/* Prepare the offline threshold - make sure we can add two blocks. */
-	vm->offline_threshold = max_t(uint64_t, 2 * memory_block_size_bytes(),
-				      VIRTIO_MEM_DEFAULT_OFFLINE_THRESHOLD);
-	/* In BBM, we also want at least two big blocks. */
-	vm->offline_threshold = max_t(uint64_t, 2 * vm->bbm.bb_size,
-				      vm->offline_threshold);
+		/* Make sure we can add two big blocks. */
+		vm->offline_threshold = max_t(uint64_t, 2 * vm->bbm.bb_size,
+					      vm->offline_threshold);
+	}
 
 	dev_info(&vm->vdev->dev, "start address: 0x%llx", vm->addr);
 	dev_info(&vm->vdev->dev, "region size: 0x%llx", vm->region_size);