Just the FAQs about Little Endian

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by Jeff Scheel, IBM Linux on Power Chief Engineer

As promised, here is my first blog post on little endian or "LE" as we call it.  Where better place to start than with a list of frequently ask questions (FAQs)?  Hopefully, you'll find this helpful.  Let me know if you have any questions I missed.

What is big endian and little endian, anyway?

In order to perform operations on data, computers routinely load and store bytes of data from and to memory, the network, and disk.  This data management generally follows one of two schemes: little endian or big endian.

Imagine the number one hundred twenty three.  When representing this number with numerals, we typically write it with the most significant digit first and the least significant digit last: 123.  This is big endian.  Mainframes and RISC architectures like POWER default to big endian when manipulating data.

Some microprocessor architectures store the numbers representing one hundred twenty three in reverse – the least significant digit first and the most significant digit last: 321.  This is little endian.  x86 architectures use little endian when storing data.

Why do people care about what endian mode their platform runs?

Most  users do not care which endian mode their platform is using.  They simply care about what applications are supported by their Linux operating systems.  Only application providers care about endianess.  For example:

• A software developer that has code manipulating data through pointer casting or bitfields would not be able to simply recompile an application for one endian mode to another.
• A user with large amounts of data stored to disk or exchanged among systems over network connections without consideration of endian schemes risks a range of application failures from very subtle to complete failures.
• A system accelerator programmer (GPU or FPGA) who needs to share memory with applications running in the system processor must share data in an pre-determined endianness for correct application functionality.

Why is Linux on Power transitioning from big endian to little endian?

The Power architecture is bi-endian in that it supports accessing data in both little endian and big endian modes. Although Power already has Linux distributions and supporting applications that run in big endian mode, the Linux application ecosystem for x86 platforms is much larger and Linux on x86 uses little endian mode.  Numerous clients, software partners, and IBM’s own software developers have told us that porting their software to Power becomes simpler if the Linux environment on Power supports little endian mode, more closely matching the environment provided by Linux on x86.  This new level of support will lower the barrier to entry for porting Linux on x86 software to Linux on Power.

Which Linux distributions will support little endian on Power?

So far, only Canonical’s Ubuntu Server 14.04 distribution supports little endian on Power.  Plans are underway in the community distributions of Debian and openSUSE for little endian releases.

Additionally, SUSE has stated publicly that SLES 12 will be little endian when it becomes available.  See SUSE Conversations for more information.

Red Hat has not yet publicly disclosed their plans around a little endian operating systems  However, work to create a ppc64le architecture has started in the Fedora.

Which Linux distributions will support big endian on Power?

It is IBM's understanding that Red Hat and SUSE will continue to support their existing big endian releases on Power for their full product lifecycles.

While SUSE has announced their plans to transition their distribution to little endian (see above), Red Hat has not disclosed anything.  The newly available Red Hat Enterprise Linux 7 operates in big endian mode on Power.  Specifics about the transition to little endian will be decided and disclosed by Red Hat.

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

The existing PowerLinux application portfolio supports only big endian modes today. Open source applications have begun extending their support to little endian mode on Power Systems.  Existing third party and IBM applications will likely migrate more slowly and deliberately.  As such, Power hardware will support both endian modes for the foreseeable future so that existing Linux applications optimized for a big endian platform will continue to run unchanged while new applications optimized to little endian mode are added.

Can applications compiled for x86 (Windows or Linux) run without change on little endian Power?

Because the x86 and Power processors use different instruction set architectures (ISAs) – the binary executable known to the processor – compiled applications will need at least a recompile on the new platform.  Whether source code changes are required depends on how many optimizations have been made in the application source – such as the use of assembler language and any assumptions about page size or cache line size, etc.

However, interpreted applications such as those in Java, perl, python, php, ruby and others should be capable of migrating with little to no change.

Does this transition affect application ecosystems for AIX or IBM i?

No, there will be no effect on AIX or IBM i application environments as a result of this change.

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

Each Linux distribution will support a particular endian mode, little or big.  Applications always certify to specific distributions.  As such, endian mode decisions should be transparent to the end user.  Customers should not have to consider endianess in their application choice.

If one requires different Linux distributions or the same distribution at different releases on a single server, then Power Systems virtualization (LPARs or VMs) allows customers to run applications supported by a big endian Linux distribution like RHEL6 as well as applications supported by a little endian distribution like Canonical’s Ubuntu Server at the same time. However, concurrent  little endian and big endian support on the same server will not be available until a future date.  See more details in the questions below.

Which POWER processors support little endian mode?

The POWER8 processor is the first processor to support little endian and big endian modes equivalently.  Although previous generations of the POWER processors had basic little endian functionality, they did not fully implement the necessary instructions in such a way to enable enterprise operating system offerings.

Where can little endian distributions run on Power?

When IBM announced POWER8 in April 2014, little endian (LE) operating systems were initially supported as KVM guests.  Further, KVM support was limited to only include all LE or all big endian (BE) guests.  In coming releases, IBM expects to support concurrent LE and BE guests in KVM, as well as the support of LE guests on PowerVM.

Do 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 SPR (special purpose register) settings.  While this could technically support the 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 have allowed for mixed environments of operating system levels and types.  This same isolation mechanism applies to big and little endian operating systems.  However, in implementing the initial releases of little endian, IBM has introduced some short-term limitations on where LE operating systems can run.  Over time, these will be removed and both KVM and PowerVM will support concurrent mixing of LE and BE operating systems.

See the previous question for more information.

Does PowerVM support little endian operating systems?

While the POWER8 systems support little endian (LE) mode, IBM has not yet completed the software development and testing to enable LE operating systems on PowerVM.  The outlook is that this function will be delivered around  mid-2015.  When this capability is delivered, PowerVM will support the mixing of both big endian (BE) and LE operating systems.  This enablement will also enable the running of LE operating systems on the Power Integrated Facilities for Linux (IFLs).

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

Testing has not yet completed to enable the mixing of little endian (LE) and big endian (BE) guests for KVM.  Until this completes, IBM supports guests of the same type – all LE or all BE.

IBM hopes to support mixing of guest types around mid-2015.

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.

January 23, 2015 - Author's update

A couple noteworthy activities have occurred since this blog was originally published.

1. A little endian (LE) version of RHEL 7.1 has been released in beta form.  This announcement indicates that RHEL 7 updates will have both the existing big endian (BE) offering and a new LE offering.  For more information about the beta, see the RHEL 7.1 beta announcement information.  This means that all three Linux on Power distribution partners -- SUSE, Canonical, and now Red Hat -- have LE operating systems.
2. IBM PowerKVM now supports the mixture of BE and LE guests beginning with the 2.1.1 update in October 2014.  This was a subtle change that is hard to find in documentation.
3. Support for LE operating systems on PowerVM continues to make progress toward a delivery sooner versus later this year.  When this is delivered, the mixing of BE and LE logical partitions will be supported.

Additionally, the following question keeps being asked and needs it's own FAQ:

Can I run x86 Linux applications on LE Linux on Power operating systems unchanged?

If your application was written in a dynamic language, it is highly portable and often migrates to BE and LE Linux on Power operating system environments without change.  Examples include applications written in Java, php, perl, python, etc.

If your application was written in a compiled language like C/C++, it must be recompiled on Power in both the BE and LE operating systems.  Applications migrating from x86 Linux onto an LE Linux operating system on Power will migrate without concern for data layout (endianness).  Applications migrating onto BE operating systems need to be reviewed for consist data access, especially if they will share data using disk or networking with LE systems.