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>
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Leonardo Garcia
Cédric Le Goater
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pSeries family boards (``pseries``)
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===================================
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The Power machine para-virtualized environment described by the `Linux on Power
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Architecture Reference document (LoPAR)
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<https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200812.pdf>`_
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is called pSeries. This environment is also known as sPAPR, System p guests, or
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simply Power Linux guests (although it is capable of running other operating
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systems, such as AIX).
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Even though pSeries is designed to behave as a guest environment, it is also
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capable of acting as a hypervisor OS, providing, on that role, nested
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virtualization capabilities.
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Supported devices
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-----------------
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* Multi processor support for many Power processors generations: POWER7,
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POWER7+, POWER8, POWER8NVL, POWER9, and Power10. Support for POWER5+ exists,
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but its state is unknown.
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* Interrupt Controller, XICS (POWER8) and XIVE (POWER9 and Power10)
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* vPHB PCIe Host bridge.
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* vscsi and vnet devices, compatible with the same devices available on a
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PowerVM hypervisor with VIOS managing LPARs.
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* Virtio based devices.
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* PCIe device pass through.
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Missing devices
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---------------
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* SPICE support.
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Firmware
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--------
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`SLOF <https://github.com/aik/SLOF>`_ (Slimline Open Firmware) is an
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implementation of the `IEEE 1275-1994, Standard for Boot (Initialization
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Configuration) Firmware: Core Requirements and Practices
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<https://standards.ieee.org/standard/1275-1994.html>`_.
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QEMU includes a prebuilt image of SLOF which is updated when a more recent
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version is required.
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Build directions
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----------------
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.. code-block:: bash
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./configure --target-list=ppc64-softmmu && make
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Running instructions
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--------------------
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Someone can select the pSeries machine type by running QEMU with the following
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options:
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.. code-block:: bash
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qemu-system-ppc64 -M pseries <other QEMU arguments>
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sPAPR devices
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-------------
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The sPAPR specification defines a set of para-virtualized devices, which are
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also supported by the pSeries machine in QEMU and can be instantiated with the
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``-device`` option:
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* ``spapr-vlan`` : a virtual network interface.
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* ``spapr-vscsi`` : a virtual SCSI disk interface.
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* ``spapr-rng`` : a pseudo-device for passing random number generator data to the
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guest (see the `H_RANDOM hypercall feature
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<https://wiki.qemu.org/Features/HRandomHypercall>`_ for details).
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* ``spapr-vty``: a virtual teletype.
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* ``spapr-pci-host-bridge``: a PCI host bridge.
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* ``tpm-spapr``: a Trusted Platform Module (TPM).
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* ``spapr-tpm-proxy``: a TPM proxy.
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These are compatible with the devices historically available for use when
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running the IBM PowerVM hypervisor with LPARs.
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However, since these devices have originally been specified with another
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hypervisor and non-Linux guests in mind, you should use the virtio counterparts
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(virtio-net, virtio-blk/scsi and virtio-rng for instance) if possible instead,
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since they will most probably give you better performance with Linux guests in a
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QEMU environment.
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The pSeries machine in QEMU is always instantiated with the following devices:
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* A NVRAM device (``spapr-nvram``).
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* A virtual teletype (``spapr-vty``).
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* A PCI host bridge (``spapr-pci-host-bridge``).
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Hence, it is not needed to add them manually, unless you use the ``-nodefaults``
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command line option in QEMU.
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In the case of the default ``spapr-nvram`` device, if someone wants to make the
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contents of the NVRAM device persistent, they will need to specify a PFLASH
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device when starting QEMU, i.e. either use
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``-drive if=pflash,file=<filename>,format=raw`` to set the default PFLASH
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device, or specify one with an ID
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(``-drive if=none,file=<filename>,format=raw,id=pfid``) and pass that ID to the
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NVRAM device with ``-global spapr-nvram.drive=pfid``.
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sPAPR specification
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^^^^^^^^^^^^^^^^^^^
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The main source of documentation on the sPAPR standard is the `Linux on Power
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Architecture Reference document (LoPAR)
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<https://openpowerfoundation.org/wp-content/uploads/2020/07/LoPAR-20200812.pdf>`_.
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However, documentation specific to QEMU's implementation of the specification
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can also be found in QEMU documentation:
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.. toctree::
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:maxdepth: 1
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../../specs/ppc-spapr-numa.rst
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../../specs/ppc-spapr-xive.rst
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Other documentation available in QEMU docs directory:
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* Hypervisor calls (a.k.a. hcalls) (``docs/specs/ppc-spapr-hcalls.txt``).
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* Hot plug (``/docs/specs/ppc-spapr-hotplug.txt``).
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* Hypervisor calls needed by the Ultravisor
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(``/docs/specs/ppc-spapr-uv-hcalls.txt``).
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Switching between the KVM-PR and KVM-HV kernel module
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-----------------------------------------------------
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Currently, there are two implementations of KVM on Power, ``kvm_hv.ko`` and
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``kvm_pr.ko``.
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If a host supports both KVM modes, and both KVM kernel modules are loaded, it is
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possible to switch between the two modes with the ``kvm-type`` parameter:
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* Use ``qemu-system-ppc64 -M pseries,accel=kvm,kvm-type=PR`` to use the
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``kvm_pr.ko`` kernel module.
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* Use ``qemu-system-ppc64 -M pseries,accel=kvm,kvm-type=HV`` to use ``kvm_hv.ko``
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instead.
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KVM-PR
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^^^^^^
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KVM-PR uses the so-called **PR**\ oblem state of the PPC CPUs to run the guests,
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i.e. the virtual machine is run in user mode and all privileged instructions
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trap and have to be emulated by the host. That means you can run KVM-PR inside
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a pSeries guest (or a PowerVM LPAR for that matter), and that is where it has
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originated, as historically (prior to POWER7) it was not possible to run Linux
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on hypervisor mode on a Power processor (this function was restricted to
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PowerVM, the IBM proprietary hypervisor).
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Because all privileged instructions are trapped, guests that use a lot of
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privileged instructions run quite slow with KVM-PR. On the other hand, because
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of that, this kernel module can run on pretty much every PPC hardware, and is
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able to emulate a lot of guests CPUs. This module can even be used to run other
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PowerPC guests like an emulated PowerMac.
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As KVM-PR can be run inside a pSeries guest, it can also provide nested
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virtualization capabilities (i.e. running a guest from within a guest).
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It is important to notice that, as KVM-HV provides a much better execution
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performance, maintenance work has been much more focused on it in the past
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years. Maintenance for KVM-PR has been minimal.
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In order to run KVM-PR guests with POWER9 processors, someone will need to start
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QEMU with ``kernel_irqchip=off`` command line option.
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KVM-HV
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^^^^^^
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KVM-HV uses the hypervisor mode of more recent Power processors, that allow
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access to the bare metal hardware directly. Although POWER7 had this capability,
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it was only starting with POWER8 that this was officially supported by IBM.
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Originally, KVM-HV was only available when running on a PowerNV platform (a.k.a.
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Power bare metal). Although it runs on a PowerNV platform, it can only be used
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to start pSeries guests. As the pSeries guest doesn't have access to the
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hypervisor mode of the Power CPU, it wasn't possible to run KVM-HV on a guest.
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This limitation has been lifted, and now it is possible to run KVM-HV inside
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pSeries guests as well, making nested virtualization possible with KVM-HV.
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As KVM-HV has access to privileged instructions, guests that use a lot of these
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can run much faster than with KVM-PR. On the other hand, the guest CPU has to be
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of the same type as the host CPU this way, e.g. it is not possible to specify an
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embedded PPC CPU for the guest with KVM-HV. However, there is at least the
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possibility to run the guest in a backward-compatibility mode of the previous
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CPUs generations, e.g. you can run a POWER7 guest on a POWER8 host by using
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``-cpu POWER8,compat=power7`` as parameter to QEMU.
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Modules support
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---------------
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As noticed in the sections above, each module can run in a different
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environment. The following table shows with which environment each module can
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run. As long as you are in a supported environment, you can run KVM-PR or KVM-HV
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nested. Combinations not shown in the table are not available.
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+--------------+------------+------+-------------------+----------+--------+
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| Platform | Host type | Bits | Page table format | KVM-HV | KVM-PR |
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+==============+============+======+===================+==========+========+
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| PowerNV | bare metal | 32 | hash | no | yes |
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| | | +-------------------+----------+--------+
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| | | | radix | N/A | N/A |
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| | +------+-------------------+----------+--------+
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| | | 64 | hash | yes | yes |
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| | | +-------------------+----------+--------+
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| | | | radix | yes | no |
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+--------------+------------+------+-------------------+----------+--------+
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| pSeries [1]_ | PowerNV | 32 | hash | no | yes |
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| | | +-------------------+----------+--------+
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| | | | radix | N/A | N/A |
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| | +------+-------------------+----------+--------+
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| | | 64 | hash | no | yes |
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| | | +-------------------+----------+--------+
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| | | | radix | yes [2]_ | no |
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| +------------+------+-------------------+----------+--------+
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| | PowerVM | 32 | hash | no | yes |
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| | | +-------------------+----------+--------+
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| | | | radix | N/A | N/A |
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| | +------+-------------------+----------+--------+
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| | | 64 | hash | no | yes |
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| | | +-------------------+----------+--------+
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| | | | radix [3]_ | no | yes |
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+--------------+------------+------+-------------------+----------+--------+
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.. [1] On POWER9 DD2.1 processors, the page table format on the host and guest
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must be the same.
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.. [2] KVM-HV cannot run nested on POWER8 machines.
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.. [3] Introduced on Power10 machines.
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Maintainer contact information
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------------------------------
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Cédric Le Goater <clg@kaod.org>
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Daniel Henrique Barboza <danielhb413@gmail.com>
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