Re: [PATCH] docs: Introducing pseries documentation.

[David Gibson]

On Wed, Nov 17, 2021 at 05:14:30PM -0300, lagarcia@linux.ibm.com wrote:
> From: Leonardo Garcia <lagarcia@br.ibm.com>
> 
> 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>

LGTM,

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Couple of minor points below.

> ---
>  docs/system/ppc/pseries.rst | 185 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 185 insertions(+)
> 
> diff --git a/docs/system/ppc/pseries.rst b/docs/system/ppc/pseries.rst
> index 932d4dd17d..2de3fb4d51 100644
> --- a/docs/system/ppc/pseries.rst
> +++ b/docs/system/ppc/pseries.rst
> @@ -1,12 +1,197 @@
>  pSeries family boards (``pseries``)
>  ===================================
>  
> +The Power machine virtualized environment described by the `Linux on Power
> +Architecture Reference document (LoPAR)
> +<https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200611.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: POWER5+,
> +   POWER7, POWER7+, POWER8, POWER8NVL, Power9, and Power10 (there is no 
> support
> +   for POWER6 processors).

I wouldn't trust the POWER5+ cpu emulation with pseries; only POWER7
and later has been tested at all.  Actually.. I wouldn't trust the
POWER5+ CPU emulation much at all, if it's been tested, it's not for a
long, long time.

> + * 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).
> +
> +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) 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 a NVRAM device
> +(``spapr-nvram``), so it is not needed to add this manually. However, 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``.
> +
> +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).

Might be worth mentioning that KVM-PR is barely maintained now, and it
doesn't support all the privileged instructions of the later
processors (I'm pretty sure POWER9 doesn't work there, for example).

> +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 [*]_ | powernv    | 32   | hash              | no       | yes    |
> +|              |            |      +-------------------+----------+--------+
> +|              |            |      | radix             | N/A      | N/A    |
> +|              |            +------+-------------------+----------+--------+
> +|              |            | 64   | hash              | no       | yes    |
> +|              |            |      +-------------------+----------+--------+
> +|              |            |      | radix             | yes [*]_ | no     |
> +|              +------------+------+-------------------+----------+--------+
> +|              | PowerVM    | 32   | hash              | no       | yes    |
> +|              |            |      +-------------------+----------+--------+
> +|              |            |      | radix             | N/A      | N/A    |
> +|              |            +------+-------------------+----------+--------+
> +|              |            | 64   | hash              | no       | yes    |
> +|              |            |      +-------------------+----------+--------+
> +|              |            |      | radix [*]_        | no       | yes    |
> ++--------------+------------+------+-------------------+----------+--------+
> +
> +.. [*] On POWER9 DD2.1 processors, the page table format on the host and 
> guest
> +   must be the same.
> +
> +.. [*] KVM-HV cannot run nested on POWER8 machines.
> +
> +.. [*] Introduced on Power10 machines.
> +
> +Maintainer contact information
> +------------------------------
> +
> +Cédric Le Goater <clg@kaod.org>
> +
> +Daniel Henrique Barboza <danielhb413@gmail.com>
> \ No newline at end of file

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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