As we approach 2025, hopefully none of you are still running x86 32-bit kernels / 32-bit OS software on x86_64 processors, but should you still be into that, there are improvements on the way.

Running x86 32-bit kernels/software on x86_64 hardware hasn't been too popular since the netbook days with their limited RAM capacities... And thankfully most Linux distributions have begun phasing out their 32-bit kernel / OS image support and other 32-bit software needed aside from better compatibility when running a x86_64 distribution. But after recently working on 32-bit ARM clean-ups within the Linux kernel, longtime Linux developer Arnd Bergmann has taken to cleaning up the x86 32-bit kernel code used when running on x86_64 Intel/AMD processors.

Bergmann explained in the [1]x86: 32-bit cleanups patch series this morning:

"Primarily this is about running 32-bit kernels on 64-bit hardware, which usually works but should probably be discouraged more clearly by only providing support for features that are used on real 32-bit hardware:

I found only a few 2003-era high-end servers (HP DL740 and DL760 G2) that were the only possible remaining uses of HIGHMEM64G and BIGSMP after the removal of 32-bit NUMA machines in 2014. Similary, there is only one generation of hardware with support for VT-x. All these features can be removed without hurting users.

In the CPU selection, building a 32-bit kernel optimized for AMD K8 or Intel Core2 is anachronistic, so instead only 32-bit CPU types need to be offered as optimization targets. The "generic" target on 64-bit turned out to be slightly broken, so I included a fix for that as well, replacing the compiler default target with an intentional selection between the useful levels."

The 11 patches add 185 new lines of code while stripping away 1,216 lines of code for those interested in x86 kernels on x86_64 hardware.

Notable among the patches is [2]ending 32-bit KVM host support on the basis of:

"There are very few 32-bit machines that support KVM, the main exceptions are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom Z5xx "Silverthorne" from 2008 that were all release just before their 64-bit counterparts.

Using KVM as a host on a 64-bit CPU using a 32-bit kernel generally works fine, but is rather pointless since 64-bit kernels are much better supported and deal better with the memory requirements of VM guests.

Drop all the 32-bit-only portions and the "#ifdef CONFIG_X86_64" checks of the x86 KVM code and add a Kconfig dependency to only allow building this on 64-bit kernels.

Support for 32-bit guests is of course untouched by this."

Xeon LVs were fascinating back in the day (2006), but understandable in 2025 to be clearing out 32-bit KVM host support while retaining 32-bit guest OS compatibility.



[1] https://lore.kernel.org/lkml/20241204103042.1904639-1-arnd@kernel.org/

[2] https://lore.kernel.org/lkml/20241204103042.1904639-12-arnd@kernel.org/