Machines That Share a Common CPU Module

Hardware vendors often ask how best to support ONIE on a family of machines that all share a common CPU module. This section describes infrastructure available in ONIE to help with this reality.


The infrastructure described in this section first became available with the 2017.11 release.

The hardware systems consist of a pluggable CPU module and a “base board” that contains the switching ASIC silicon and front panel ports.

The CPU module contains the main CPU subsystems, including the CPU, DRAM, storage (like mSATA and nvme m.2), CPLDs, eth0, serial console, etc.

This design makes great sense for a hardware manufacturer – they can mass produce the CPU module independently of the base boards, allowing them to support a variety of switching ASICs with a single CPU module.

The question in an ONIE environment is “how to identify the machine?” when the CPU module is common. The hardware vendors would like to build ONIE once, just for the CPU module. It complicates their life to tailor an ONIE build for each base board variant.

At the same time NOS vendors need a clean way to identify the system. If 10 different systems with a common CPU module all report “x86_64-vendor_common_cpuXYZ-r0” for 10 different base boards that is not helpful. It complicates the NOS vendor’s life to add vendor and platform specific “peek and poke” code to their installers to figure out what the base board is.

At a hardware level, the base board identifying information is typically contained in the ONIE EEPROM, using one of the optional TLV types like “Part Number”(0x22) or “Product Name”(0x21). Sometimes the information is contained in a CPLD. The details of the identifying information is not so important for this discussion.

One ONIE Image for the CPU Module

The idea is to have a single ONIE “machine” for the CPU module and detect at run time what the baseboard is.

The platform identification originates from /etc/machine.conf, a portion of which is now derived at run time. Along with this a new configuration variable, onie_build_machine is introduced.

Changes to /etc/machine.conf

Previously this file was entirely built at compile time. This file is now a small wrapper script that sources two new files, /etc/machine-build.conf and /etc/machine-live.conf.

The contents of /etc/machine.conf looks like:

# Source build-time machine configuration
. /etc/machine-build.conf

# Source run-time machine configuration if available
[ -r /etc/machine-live.conf ] && . /etc/machine-live.conf

# Use onie_machine if set, otherwise use build_machine

  • /etc/machine-build.conf – this file is completely built at compile time and only contains information known at build time. This file introduces a new configuration variable, onie_build_machine, which is the ONIE machine specified at compile time. For example this would be the ONIE machine name of the common CPU module.
  • /etc/machine-live.conf – this file is built at run time. A boot time init script sources a platform specific file if it exists, executes the gen_live_config() function and stores the output in /etc/machine-live.conf. By default this function outputs nothing, but a platform can override it produce a runtime ONIE machine name.

Using this mechanism, a machine can redefine onie_machine and onie_switch_asic at run time. To do this, a machine defines a small script in the source tree at machine/<vendor>/rootconf/sysroot-lib-onie/gen-config-platform, which include a definition of the gen_live_config() function.

If a machine does not define gen-config-platform, i.e. /etc/machine-live.conf is empty, then the contents of onie_build_machine is used to set onie_machine. This is backwardly compatible with how ONIE worked previously.

How This Affects ONIE Updates

When ONIE updates itself, it will now check that the running onie_build_machine matches the onie_build_machine of the proposed update image. Previously it checked that onie_machine matched between runtime and the image.

Working Example

For an example look at the kvm_x86_64 virtual machine. This machine contains an example sysroot-lib-onie/gen-config-platform file that can dynamically generate different run time machine names based on the environment.

To try out the kvm_x86_64 example, set the live_machine and live_asic variables on the GRUB command line when booting ONIE. At boot time the kvm_x86_64 machine detects these and dynamically generates a run time machine name.


This is just a toy example. For a real hardware platform, the base board identifying information is typically contained in the ONIE EEPROM, using one of the optional TLV types like “Part Number”(0x22) or “Product Name”(0x21). Sometimes the information is contained in a CPLD.