doc: Add initial documentation for iscsi hotplug.

Change-Id: I432b83c9bf4a39cd061579d0318b91903299e080
Signed-off-by: Cunyin Chang <cunyin.chang@intel.com>

doc/Doxyfile

@@ -768,6 +768,7 @@ INPUT                  = ../include/spdk \
                          ioat/index.md \
                          iscsi/index.md \
                          iscsi/getting_started.md \
+                         iscsi/hotplug.md \
                          nvme/index.md \
                          nvme/async_completion.md \
                          nvme/fabrics.md \

doc/iscsi/hotplug.md

@@ -0,0 +1,21 @@
+# iSCSI Hotplug {#iscsi_hotplug}
+
+At the iSCSI level, we provide the following support for Hotplug:
+
+1. bdev/nvme:
+At the bdev/nvme level, we start one hotplug monitor which will call
+spdk_nvme_probe() periodically to get the hotplug events. We provide the
+private attach_cb and remove_cb for spdk_nvme_probe(). For the attach_cb,
+we will create the block device base on the NVMe device attached, and for the
+remove_cb, we will unregister the block device, which will also notify the
+upper level stack (for iSCSI target, the upper level stack is scsi/lun) to
+handle the hot-remove event.
+
+2. scsi/lun:
+When the LUN receive the hot-remove notification from block device layer,
+the LUN will be marked as removed, and all the IOs after this point will
+return with check condition status. Then the LUN starts one poller which will
+wait for all the commands which have already been submitted to block device to
+return back; after all the commands return back, the LUN will be deleted.
+
+@sa spdk_nvme_probe

doc/iscsi/index.md

@@ -1,3 +1,4 @@
 # iSCSI Target {#iscsi}
 
 - @ref iscsi_getting_started
+- @ref iscsi_hotplug