More FAQs about Little Endian: An Update

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by Jeff Scheel, IBM Distinguished Engineer, Linux Technical Strategy

Two years ago this month, I published my first blog about the Linux on Power strategic shift from big endian to little endian, titled Just the FAQs about Little Endian, and I still get questions about it today. So, I figured it was time to make updates.

The questions listed below are either updated or new. If you do not see your particular question, please read the original document before reaching out.

Which Linux distributions support little endian on Power?

All three Linux on Power partners – Canonical, Red Hat, and SUSE – offer little endian distributions.

Beginning with the 14.04 distribution, Canonical’s Ubuntu Server supports Power in little endian mode only and future release plans show this support continuing. No plans exist to provide an equivalent big endian version optimized for IBM Power Systems.

SUSE's Linux Enterprise Server (SLES) offers SLES 12 on Power only in little endian mode only. As such, customers will have to migrate from big to little endian as they upgrade from SLES 11 to SLES 12.

Since the release of Red Hat Enterprise Linux (RHEL) 7.1, Red Hat provides both little endian and big endian versions of Linux on IBM Power Systems. At this time in 2016, these products are separately licensed and non-transferable; customers should pay close attention when ordering RHEL for Power to request the desired endianness. While Red Hat's plans around releasing RHEL as a little endian only distribution remain undisclosed, customers should view RHEL 7 as the opportunity to migrate from big to little endian versions just in case the next major release ships only as a little endian product.

At this time, all community distributions of Debian, Fedora, and openSUSE offer big and little endian versions.

How long will Linux distributions continue to support big endian on Power?

It remains IBM's understanding that Red Hat and SUSE will continue to support their existing big endian releases on Power for their full product life cycles. However, customers running big endian distributions would be wise to begin planning their transitions to little endian distributions as their applications become available and time permits.

How does POWER systems support the running of mixed environments of big and little endian operating systems?

The POWER8 processor supports mixing of big and little endian memory accesses at the core level, through the use of special purpose register (SPR) settings. While this could technically support running of both big and little endian software threads, the complexity of implementing such a design point would be high. Therefore, IBM has elected to enable operating system versions as completely big endian or little endian by design.

The virtualization capabilities of the POWER platform allow for mixed environments of operating system levels and types. This same isolation mechanism applies to big and little endian operating systems with Linux and other Power operating systems such as AIX and IBM I.

Does POWER systems support running mixed environments of big and little endian operating systems in both PowerVM and PowerKVM?

As of the PowerKVM release 2.1.1 (shipped in October 2014), KVM has supported a mix of big and little endian guests running simultaneously. Further, little endian support was added to PowerVM in the spring of 2015, allowing the system to run in mixed modes. So yes, all current PowerKVM and PowerVM releases support mixing of big endian and little endian operating systems.

More details can be found in IBM Knowledge Center Supported virtualization options for POWER8 Linux on Power servers topic.

Can I run big endian applications on a little endian operating system or vice versa?

No, the operating system enablement only supports applications of the same type. As such, a little endian operating system (ppc64le or ppc64el) can only run little endian applications built for this software platform. Likewise, big endian operating systems (ppc64) only support software built for big endian.

What if I want to run a mix of big endian and little endian applications on the same Power System?

Virtualization enables mixing of big and little endian application environments on the same server. Applications of a particular operating system and endian mode must be run in a separate virtual machine (VM) or logical partition (LPAR).

See the above question about mixed big and little endian operating system environments for more explanation.

Where can little endian distributions run on Power?

Little endian distributions can run virtualized in a VM (PowerKVM or any KVM on Power distribution from the vendors) on a POWER8 S8xxL or S8xxLC model system; in an LPAR (PowerVM) on a POWER8 S8xxL, S8xx, or E8xx model system; or on bare metal (directly on the “BIOS-like” firmware that enables KVM) on S8xxL or S8xxLC models.

More details can be found in IBM Knowledge Center Supported virtualization options for POWER8 Linux on Power servers and Supported features for bare metal POWER8 Linux on Power systems topics.

Does PowerVM support little endian operating systems?

PowerVM has supported little endian operating systems since the spring of 2015.

More details can be found in IBM Knowledge Center Supported virtualization options for POWER8 Linux on Power servers topic.

Does PowerKVM support mixing of little endian and big endian operating systems?

PowerKVM has supported mixing of KVM guests or VMs since PowerKVM 2.1.1.

More details can be found in IBM Knowledge Center Supported virtualization options for POWER8 Linux on Power servers topic.

What about Linux applications that have already been optimized for big endian on Power?

IBM remains committed to transitioning the Linux on Power application ecosystem from big endian to little endian in an expeditious manner. Most IBM products have completed the transition and new products have started as little endian only.

More information about IBM Software Products can be found for each software product by using the Software Product Compatibility Reports.

Additionally, IBM continues to work both with open source communities and third party software providers to grow the Linux on Power ecosystem. While the operating system support decisions lies with the application vendor, IBM strongly encourages new providers to start as little endian so as to eliminate any transition planning and to simplify the application development process.

For a list of active open source software, reference the IBM Linux on Power Software wiki page in the Linux on Power community.