The Linux Kernel May Finally Phase Out Intel i486 CPU Support

Written by Michael Larabel in Linux Kernel on 23 October 2022 at 05:03 AM EDT. 137 Comments
Linus Torvalds has backed the idea of possibly removing Intel 486 (i486) processor support from the Linux kernel.

After the Linux kernel dropped i386 support a decade ago, i486 has been the minimum x86 processor support for the mainline Linux kernel. This latest attempt to kill off i486 support ultimately arose from Linus Torvalds himself with expressing the idea of possible requiring x86 32-bit CPUs with "cmpxchg8b" support, which would mean Pentium CPUs and later:
Maybe we should just bite the bullet, and say that we only support x86-32 with 'cmpxchg8b' (ie Pentium and later).

Get rid of all the "emulate 64-bit atomics with cli/sti, knowing that nobody has SMP on those CPU's anyway", and implement a generic x86-32 xchg() setup using that try_cmpxchg64 loop.

I think most (all?) distros already enable X86_PAE anyway, which makes that X86_CMPXCHG64 be part of the base requirement.

Not that I'm convinced most distros even do 32-bit development anyway these days.
We got rid of i386 support back in 2012. Maybe it's time to get rid of i486 support in 2022?

When potentially dropping i486 support was raised a year ago, a developer commented he still had an i486 system and claimed to still have some use out of it. But any Linux users running a modern distribution/kernel on i486 is incredibly rare.

Torvalds further wrote on the kernel mailing list continuing to back the idea of dropping i486 support from the kernel:
So I *really* don't think i486 class hardware is relevant any more. Yes, I'm sure it exists (Maciej being an example), but from a kernel development standpoint I don't think they are really relevant.

At some point, people have them as museum pieces. They might as well run museum kernels.

Moving up to requiring cmpxchg8b doesn't sound unreasonable to me.

With Linux 6.1 likely being this year's LTS kernel, hopefully Linux 6.2 will go ahead and retire the old i486 support.
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