Ingo Molnar, the maintainer of the x86 architecture, locking mechanism, and task scheduler in the Linux kernel, has proposed a set of patches to remove support for 486 processors (M486, M486SX, AMD ELAN) and early series of 586 processors from the kernel. The kernel suggests retaining only the capability to work with x86 processors that support the CX8 instruction (CMPXCHG8B) and the TSC (Time Stamp Counter) register, which appeared in Pentium CPUs.
It is noted that supporting 486 CPUs in the kernel requires maintaining code that emulates CX8 operations (compare and exchange 8 bytes) and TSC (the CPU cycle counter used by the task scheduler). This code complicates the kernel, makes maintenance difficult, and occasionally becomes a source of problems that consume developers' time to resolve. Ending support for 486 will eliminate 14,104 lines of code from the kernel, significantly simplifying some kernel functions by removing layers that emulate CX8 and TSC and allowing the removal of the math-emu library that emulates the FPU.
A day before the publication of the patches, Linus Torvalds raised the question of the appropriateness of removing 486 support during the discussion of an ongoing issue related to CX8 emulation. Linus believes it is time to abandon support for 486 CPUs and sees no reason to continue spending developers' time on resolving the problems they cause. Support for 386 processors was removed from the kernel in 2012. Participants in the discussion agree that it is now time to remove 486 CPU support. In October 2022, Linus had already published a similar proposal, but it did not progress.
In systems that still utilize 486 processors, modern Linux kernels are practically not used. Specialized variants of 486 processors for embedded systems, such as Intel Quark, support CX8 and TSC, and their modification will not be affected. Old original 486 CPUs are generally still used with outdated distributions that provide old versions of the Linux kernel. Modern Linux distributions have long ceased support for 32-bit x86 systems or have switched to using the X86_PAE option during builds, requiring support for CX8.
Source: opennet.ru
