virito-mem: document Sub Block Mode (SBM)

Let's add some documentation for the current mode - Sub Block Mode (SBM) -
to prepare for a new mode - Big Block Mode (BBM).

Follow-up patches will properly factor out the existing Sub Block Mode
(SBM) and implement Big Block Mode (BBM).
Reviewed-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20201112133815.13332-18-david@redhat.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -27,6 +27,21 @@ static bool unplug_online = true;
 module_param(unplug_online, bool, 0644);
 MODULE_PARM_DESC(unplug_online, "Try to unplug online memory");
 
+/*
+ * virtio-mem currently supports the following modes of operation:
+ *
+ * * Sub Block Mode (SBM): A Linux memory block spans 1..X subblocks (SB). The
+ *   size of a Sub Block (SB) is determined based on the device block size, the
+ *   pageblock size, and the maximum allocation granularity of the buddy.
+ *   Subblocks within a Linux memory block might either be plugged or unplugged.
+ *   Memory is added/removed to Linux MM in Linux memory block granularity.
+ *
+ * User space / core MM (auto onlining) is responsible for onlining added
+ * Linux memory blocks - and for selecting a zone. Linux Memory Blocks are
+ * always onlined separately, and all memory within a Linux memory block is
+ * onlined to the same zone - virtio-mem relies on this behavior.
+ */
+
 enum virtio_mem_mb_state {
 	/* Unplugged, not added to Linux. Can be reused later. */
 	VIRTIO_MEM_MB_STATE_UNUSED = 0,