spapr/docs: Add a few words about x-vof

The alternative small firmware needs a few words of what it can and
absolutely cannot do; this adds those words.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20220506055124.3822112-1-aik@ozlabs.ru>
[danielhb: added linebreaks before and after table]
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

docs/system/ppc/pseries.rst

@@ -32,14 +32,43 @@ Missing devices
 Firmware
 ========
 
+The pSeries platform in QEMU comes with 2 firmwares:
+
 `SLOF <https://github.com/aik/SLOF>`_ (Slimline Open Firmware) is an
 implementation of the `IEEE 1275-1994, Standard for Boot (Initialization
 Configuration) Firmware: Core Requirements and Practices
 <https://standards.ieee.org/standard/1275-1994.html>`_.
 
+SLOF performs bus scanning, PCI resource allocation, provides the client
+interface to boot from block devices and network.
+
 QEMU includes a prebuilt image of SLOF which is updated when a more recent
 version is required.
 
+VOF (Virtual Open Firmware) is a minimalistic firmware to work with
+``-machine pseries,x-vof=on``. When enabled, the firmware acts as a slim
+shim and QEMU implements parts of the IEEE 1275 Open Firmware interface.
+
+VOF does not have device drivers, does not do PCI resource allocation and
+relies on ``-kernel`` used with Linux kernels recent enough (v5.4+)
+to PCI resource assignment. It is ideal to use with petitboot.
+
+Booting via ``-kernel`` supports the following:
+
++-------------------+-------------------+------------------+
+| kernel            | pseries,x-vof=off | pseries,x-vof=on |
++===================+===================+==================+
+| vmlinux BE        |     ✓             |     ✓            |
++-------------------+-------------------+------------------+
+| vmlinux LE        |     ✓             |     ✓            |
++-------------------+-------------------+------------------+
+| zImage.pseries BE |     x             |     ✓¹           |
++-------------------+-------------------+------------------+
+| zImage.pseries LE |     ✓             |     ✓            |
++-------------------+-------------------+------------------+
+
+¹ must set kernel-addr=0
+
 Build directions
 ================