pSeries family boards (pseries) — QEMU documentation

# pSeries family boards (`pseries`)

The Power machine para-virtualized environment described by the Linux on Power Architecture Reference ([LoPAR]) document 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

The pSeries platform in QEMU comes with 2 firmwares:

SLOF (Slimline Open Firmware) is an implementation of the IEEE 1275-1994, Standard for Boot (Initialization Configuration) Firmware: Core Requirements and Practices.

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

    ./configure --target-list=ppc64-softmmu && make

## Running instructions

Someone can select the pSeries machine type by running QEMU with the following options:

    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 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 [LoPAR] document. However, documentation specific to QEMU’s implementation of the specification can also be found in QEMU documentation:

- sPAPR Dynamic Reconfiguration
- sPAPR hypervisor calls
- NUMA mechanics for sPAPR (pseries machines)
- How the pseries Linux guest calculates NUMA distances
- pseries NUMA mechanics
- Legacy (5.1 and older) pseries NUMA mechanics
- Hypervisor calls and the Ultravisor
- XIVE for sPAPR (pseries machines)

## 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 PRoblem 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.

### POWER (PAPR) Protected Execution Facility (PEF)

Protected Execution Facility (PEF), also known as Secure Guest support is a feature found on IBM POWER9 and POWER10 processors.

If a suitable firmware including an Ultravisor is installed, it adds an extra memory protection mode to the CPU. The ultravisor manages a pool of secure memory which cannot be accessed by the hypervisor.

When this feature is enabled in QEMU, a guest can use ultracalls to enter “secure mode”. This transfers most of its memory to secure memory, where it cannot be eavesdropped by a compromised hypervisor.

#### Launching

To launch a guest which will be permitted to enter PEF secure mode:

    $ qemu-system-ppc64 \
        -object pef-guest,id=pef0 \
        -machine confidential-guest-support=pef0 \
        ...

#### Live Migration

Live migration is not yet implemented for PEF guests. For consistency, QEMU currently prevents migration if the PEF feature is enabled, whether or not the guest has actually entered secure mode.

## Maintainer contact information

Cédric Le Goater <clg@kaod.org>

Daniel Henrique Barboza <danielhb413@gmail.com>

[LoPAR] (1,2)

Linux on Power Architecture Reference document (LoPAR) revision 2.9.