DOC: Usage guide for Virtio-blk blkrescan feature

This patch adds the steps required to use the newly
introduced blkrescan feature for Virtio-blk device.

Tracked-On: #3051
Signed-off-by: Vijay Dhanraj <vijay.dhanraj@intel.com>

doc/developer-guides/primer.rst

@@ -795,6 +795,35 @@ To use the Virtio-blk device, use the following command:
          nohpet console=hvc0 no_timer_check ignore_loglevel \
          log_buf_len=16M consoleblank=0 tsc=reliable" vm1
 
+Virtio-blk also supports rescan feature. Virtio-blk rescan is helpful
+when the size/path of the Virtio-blk device's backing file (on the host)
+changes and the guest needs to refresh its internal data structures to
+pick up this change. Using rescan feature is simple, add Virtio-blk
+device with dummy backend using "**nodisk**" keyword instead of a backing
+file.
+
+To create a Virtio-blk device with dummy backend,
+use the following command:
+
+.. code-block:: bash
+
+   ./acrn-dm -A -m 1168M \
+      -s 0:0,hostbridge \
+      -s 1,virtio-blk,**nodisk** \
+      -k bzImage -B "root=/dev/vda rw rootwait noxsave maxcpus=0 \
+         nohpet console=hvc0 no_timer_check ignore_loglevel \
+         log_buf_len=16M consoleblank=0 tsc=reliable" vm1
+
+After VM launch, user can replace the dummy backend file with the
+actual file using acrnctl "blkrescan" cmd as shown below,
+
+.. code-block:: bash
+
+   acrnctl blkrescan vm1 1,actual_file.img
+
+Please refer to :ref:`acrnctl` for further details on
+``acrnctl blkrescan`` command.
+
 To verify the result, you should expect the user OS to boot
 successfully.
 

tools/acrn-manager/README.rst

@@ -32,6 +32,7 @@ You can see the available ``acrnctl`` commands by running:
      suspend
      resume
      reset
+     blkrescan
    Use acrnctl [cmd] help for details
 
 Here are some usage examples:
@@ -93,6 +94,27 @@ Use the ``stop`` command to stop one or more running VM:
 
    # acrnctl stop vm-yocto vm1-14:59:30 vm-android
 
+RESCAN BLOCK DEVICE
+===================
+
+Use the ``blkrescan`` command to trigger a rescan of
+virtio-blk device by guest VM, in order to revalidate and
+update the backend file.
+
+.. code-block:: none
+
+   # acrnctl blkrescan vmname slot,newfilepath
+   vmname:     Name of VM with dummy backend file attached to virtio-blk device.
+   slot:       Slot number of the virtio-blk device.
+   newfilepath: File path for the backend of virtio-blk device.
+
+   acrnctl blkrescan vm1 6,actual_file.img
+
+.. note:: blkrescan is only supported when VM is launched with
+   empty backend file (using **nodisk**) for virtio-blk device.
+   Replacing a valid backend file is not supported and will
+   result in error.
+
 .. _acrnd:
 
 acrnd