Re: [PATCH] docs: Introducing pseries documentation.

["Leonardo Augusto Guimarães Garcia" <lagarcia@linux.ibm.com>]

On 11/17/21 20:30, David Gibson wrote:
> 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>


Thanks for the review.


Minor comments below.


>
> 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.


Sure. Removed POWER5+ from the list of supported processors and put a
note saying that the code is there but the state is unknown in v2.


>> + * 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).


I put a note on this on v2. My understanding is that POWER9 is working
now as long as you start QEMU with kernel_irqchip=off, but I agree with
you that maintenance focus hasn't been on KVM-PR anyway.


Cheers,


Leo


>> +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