docs/system/arm: xlnx-versal-virt: BBRAM and eFUSE Usage

Add BBRAM and eFUSE usage to the Xilinx Versal Virt board
document.

Signed-off-by: Tong Ho <tong.ho@xilinx.com>
Message-id: 20210917052400.1249094-10-tong.ho@xilinx.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/xlnx-versal-virt.rst

@@ -32,6 +32,8 @@ Implemented devices:
 - OCM (256KB of On Chip Memory)
 - XRAM (4MB of on chip Accelerator RAM)
 - DDR memory
+- BBRAM (36 bytes of Battery-backed RAM)
+- eFUSE (3072 bytes of one-time field-programmable bit array)
 
 QEMU does not yet model any other devices, including the PL and the AI Engine.
 
@@ -175,3 +177,50 @@ Run the following at the U-Boot prompt:
   fdt set /chosen/dom0 reg <0x00000000 0x40000000 0x0 0x03100000>
   booti 30000000 - 20000000
 
+BBRAM File Backend
+""""""""""""""""""
+BBRAM can have an optional file backend, which must be a seekable
+binary file with a size of 36 bytes or larger. A file with all
+binary 0s is a 'blank'.
+
+To add a file-backend for the BBRAM:
+
+.. code-block:: bash
+
+  -drive if=pflash,index=0,file=versal-bbram.bin,format=raw
+
+To use a different index value, N, from default of 0, add:
+
+.. code-block:: bash
+
+  -global xlnx,bbram-ctrl.drive-index=N
+
+eFUSE File Backend
+""""""""""""""""""
+eFUSE can have an optional file backend, which must be a seekable
+binary file with a size of 3072 bytes or larger. A file with all
+binary 0s is a 'blank'.
+
+To add a file-backend for the eFUSE:
+
+.. code-block:: bash
+
+  -drive if=pflash,index=1,file=versal-efuse.bin,format=raw
+
+To use a different index value, N, from default of 1, add:
+
+.. code-block:: bash
+
+  -global xlnx,efuse.drive-index=N
+
+.. warning::
+  In actual physical Versal, BBRAM and eFUSE contain sensitive data.
+  The QEMU device models do **not** encrypt nor obfuscate any data
+  when holding them in models' memory or when writing them to their
+  file backends.
+
+  Thus, a file backend should be used with caution, and 'format=luks'
+  is highly recommended (albeit with usage complexity).
+
+  Better yet, do not use actual product data when running guest image
+  on this Xilinx Versal Virt board.