docs: Introducing pseries documentation.

The purpose of this document is to substitute the content currently
available in the QEMU wiki at [0]. This initial version does contain
some additional content as well. Whenever this documentation gets
upstream and is reflected in [1], the QEMU wiki will be edited to point
to this documentation, so that we only need to keep it updated in one
place.

0. https://wiki.qemu.org/Documentation/Platforms/POWER
1. https://qemu.readthedocs.io/en/latest/system/ppc/pseries.html

Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <66b6fdde52062fdf4f4b4dc35a9f06a899c88293.1638981899.git.lagarcia@br.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

1 files changed, 226 insertions, 0 deletions

diff --git a/docs/system/ppc/pseries.rst b/docs/system/ppc/pseries.rst
index 932d4dd..0d2e4af 100644
--- a/docs/system/ppc/pseries.rst
+++ b/docs/system/ppc/pseries.rst
@@ -1,12 +1,238 @@
 pSeries family boards (``pseries``)
 ===================================
 
+The Power machine para-virtualized environment described by the `Linux on Power
+Architecture Reference document (LoPAR)
+<https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200812.pdf>`_
+is called pSeries. This environment is also known as sPAPR, System p guests, or
+simply Power Linux guests (although it is capable of running other operating
+systems, such as AIX).
+
+Even though pSeries is designed to behave as a guest environment, it is also
+capable of acting as a hypervisor OS, providing, on that role, nested
+virtualization capabilities.
+
 Supported devices
 -----------------
 
+ * Multi processor support for many Power processors generations: POWER7,
+   POWER7+, POWER8, POWER8NVL, POWER9, and Power10. Support for POWER5+ exists,
+   but its state is unknown.
+ * Interrupt Controller, XICS (POWER8) and XIVE (POWER9 and Power10)
+ * vPHB PCIe Host bridge.
+ * vscsi and vnet devices, compatible with the same devices available on a
+   PowerVM hypervisor with VIOS managing LPARs.
+ * Virtio based devices.
+ * PCIe device pass through.
+
 Missing devices
 ---------------
 
+ * SPICE support.
 
 Firmware
 --------
+
+`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>`_.
+
+QEMU includes a prebuilt image of SLOF which is updated when a more recent
+version is required.
+
+Build directions
+----------------
+
+.. code-block:: bash
+
+  ./configure --target-list=ppc64-softmmu && make
+
+Running instructions
+--------------------
+
+Someone can select the pSeries machine type by running QEMU with the following
+options:
+
+.. code-block:: bash
+
+  qemu-system-ppc64 -M pseries <other QEMU arguments>
+
+sPAPR devices
+-------------
+
+The sPAPR specification defines a set of para-virtualized devices, which are
+also supported by the pSeries machine in QEMU and can be instantiated with the
+``-device`` option:
+
+* ``spapr-vlan`` : a virtual network interface.
+* ``spapr-vscsi`` : a virtual SCSI disk interface.
+* ``spapr-rng`` : a pseudo-device for passing random number generator data to the
+  guest (see the `H_RANDOM hypercall feature
+  <https://wiki.qemu.org/Features/HRandomHypercall>`_ for details).
+* ``spapr-vty``: a virtual teletype.
+* ``spapr-pci-host-bridge``: a PCI host bridge.
+* ``tpm-spapr``: a Trusted Platform Module (TPM).
+* ``spapr-tpm-proxy``: a TPM proxy.
+
+These are compatible with the devices historically available for use when
+running the IBM PowerVM hypervisor with LPARs.
+
+However, since these devices have originally been specified with another
+hypervisor and non-Linux guests in mind, you should use the virtio counterparts
+(virtio-net, virtio-blk/scsi and virtio-rng for instance) if possible instead,
+since they will most probably give you better performance with Linux guests in a
+QEMU environment.
+
+The pSeries machine in QEMU is always instantiated with the following devices:
+
+* A NVRAM device (``spapr-nvram``).
+* A virtual teletype (``spapr-vty``).
+* A PCI host bridge (``spapr-pci-host-bridge``).
+
+Hence, it is not needed to add them manually, unless you use the ``-nodefaults``
+command line option in QEMU.
+
+In the case of the default ``spapr-nvram`` device, if someone wants to make the
+contents of the NVRAM device persistent, they will need to specify a PFLASH
+device when starting QEMU, i.e. either use
+``-drive if=pflash,file=<filename>,format=raw`` to set the default PFLASH
+device, or specify one with an ID
+(``-drive if=none,file=<filename>,format=raw,id=pfid``) and pass that ID to the
+NVRAM device with ``-global spapr-nvram.drive=pfid``.
+
+sPAPR specification
+^^^^^^^^^^^^^^^^^^^
+
+The main source of documentation on the sPAPR standard is the `Linux on Power
+Architecture Reference document (LoPAR)
+<https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200812.pdf>`_.
+However, documentation specific to QEMU's implementation of the specification
+can  also be found in QEMU documentation:
+
+.. toctree::
+   :maxdepth: 1
+
+   ../../specs/ppc-spapr-numa.rst
+   ../../specs/ppc-spapr-xive.rst
+
+Other documentation available in QEMU docs directory:
+
+* Hypervisor calls (a.k.a. hcalls) (``docs/specs/ppc-spapr-hcalls.txt``).
+* Hot plug (``/docs/specs/ppc-spapr-hotplug.txt``).
+* Hypervisor calls needed by the Ultravisor
+  (``/docs/specs/ppc-spapr-uv-hcalls.txt``).
+
+Switching between the KVM-PR and KVM-HV kernel module
+-----------------------------------------------------
+
+Currently, there are two implementations of KVM on Power, ``kvm_hv.ko`` and
+``kvm_pr.ko``.
+
+
+If a host supports both KVM modes, and both KVM kernel modules are loaded, it is
+possible to switch between the two modes with the ``kvm-type`` parameter:
+
+* Use ``qemu-system-ppc64 -M pseries,accel=kvm,kvm-type=PR`` to use the
+  ``kvm_pr.ko`` kernel module.
+* Use ``qemu-system-ppc64 -M pseries,accel=kvm,kvm-type=HV`` to use ``kvm_hv.ko``
+  instead.
+
+KVM-PR
+^^^^^^
+
+KVM-PR uses the so-called **PR**\ oblem state of the PPC CPUs to run the guests,
+i.e. the virtual machine is run in user mode and all privileged instructions
+trap and have to be emulated by the host. That means you can run KVM-PR inside
+a pSeries guest (or a PowerVM LPAR for that matter), and that is where it has
+originated, as historically (prior to POWER7) it was not possible to run Linux
+on hypervisor mode on a Power processor (this function was restricted to
+PowerVM, the IBM proprietary hypervisor).
+
+Because all privileged instructions are trapped, guests that use a lot of
+privileged instructions run quite slow with KVM-PR. On the other hand, because
+of that, this kernel module can run on pretty much every PPC hardware, and is
+able to emulate a lot of guests CPUs. This module can even be used to run other
+PowerPC guests like an emulated PowerMac.
+
+As KVM-PR can be run inside a pSeries guest, it can also provide nested
+virtualization capabilities (i.e. running a guest from within a guest).
+
+It is important to notice that, as KVM-HV provides a much better execution
+performance, maintenance work has been much more focused on it in the past
+years. Maintenance for KVM-PR has been minimal.
+
+In order to run KVM-PR guests with POWER9 processors, someone will need to start
+QEMU with ``kernel_irqchip=off`` command line option.
+
+KVM-HV
+^^^^^^
+
+KVM-HV uses the hypervisor mode of more recent Power processors, that allow
+access to the bare metal hardware directly. Although POWER7 had this capability,
+it was only starting with POWER8 that this was officially supported by IBM.
+
+Originally, KVM-HV was only available when running on a PowerNV platform (a.k.a.
+Power bare metal). Although it runs on a PowerNV platform, it can only be used
+to start pSeries guests. As the pSeries guest doesn't have access to the
+hypervisor mode of the Power CPU, it wasn't possible to run KVM-HV on a guest.
+This limitation has been lifted, and now it is possible to run KVM-HV inside
+pSeries guests as well, making nested virtualization possible with KVM-HV.
+
+As KVM-HV has access to privileged instructions, guests that use a lot of these
+can run much faster than with KVM-PR. On the other hand, the guest CPU has to be
+of the same type as the host CPU this way, e.g. it is not possible to specify an
+embedded PPC CPU for the guest with KVM-HV. However, there is at least the
+possibility to run the guest in a backward-compatibility mode of the previous
+CPUs generations, e.g. you can run a POWER7 guest on a POWER8 host by using
+``-cpu POWER8,compat=power7`` as parameter to QEMU.
+
+Modules support
+---------------
+
+As noticed in the sections above, each module can run in a different
+environment. The following table shows with which environment each module can
+run. As long as you are in a supported environment, you can run KVM-PR or KVM-HV
+nested. Combinations not shown in the table are not available.
+
++--------------+------------+------+-------------------+----------+--------+
+| Platform     | Host type  | Bits | Page table format | KVM-HV   | KVM-PR |
++==============+============+======+===================+==========+========+
+| PowerNV      | bare metal | 32   | hash              | no       | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix             | N/A      | N/A    |
+|              |            +------+-------------------+----------+--------+
+|              |            | 64   | hash              | yes      | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix             | yes      | no     |
++--------------+------------+------+-------------------+----------+--------+
+| pSeries [1]_ | PowerNV    | 32   | hash              | no       | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix             | N/A      | N/A    |
+|              |            +------+-------------------+----------+--------+
+|              |            | 64   | hash              | no       | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix             | yes [2]_ | no     |
+|              +------------+------+-------------------+----------+--------+
+|              | PowerVM    | 32   | hash              | no       | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix             | N/A      | N/A    |
+|              |            +------+-------------------+----------+--------+
+|              |            | 64   | hash              | no       | yes    |
+|              |            |      +-------------------+----------+--------+
+|              |            |      | radix [3]_        | no       | yes    |
++--------------+------------+------+-------------------+----------+--------+
+
+.. [1] On POWER9 DD2.1 processors, the page table format on the host and guest
+   must be the same.
+
+.. [2] KVM-HV cannot run nested on POWER8 machines.
+
+.. [3] Introduced on Power10 machines.
+
+Maintainer contact information
+------------------------------
+
+Cédric Le Goater <clg@kaod.org>
+
+Daniel Henrique Barboza <danielhb413@gmail.com>