linux-kernel - Re: [patch 5/9] x86: Cure per CPU madness on UP

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Message-Id: <dd494544-6dbf-4e83-8060-1f8b7b07a8c4@app.fastmail.com>
Date: Sat, 16 Mar 2024 22:34:55 +0100
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Linus Torvalds" <torvalds@...uxfoundation.org>,
 "Thomas Gleixner" <tglx@...utronix.de>
Cc: "Guenter Roeck" <linux@...ck-us.net>, LKML <linux-kernel@...r.kernel.org>,
 x86@...nel.org, "Uros Bizjak" <ubizjak@...il.com>,
 linux-sparse@...r.kernel.org, "kernel test robot" <lkp@...el.com>,
 oe-kbuild-all@...ts.linux.dev
Subject: Re: [patch 5/9] x86: Cure per CPU madness on UP

On Sat, Mar 16, 2024, at 02:23, Linus Torvalds wrote:
> On Fri, 15 Mar 2024 at 18:11, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> You wish. We still support 486 and some of the still produced 486 clones
>> do not have a local APIC.
>
> Ouch. I was _sure_ we had dropped i486 support too due to cmpxchg8b.
>
> But apparently that was just a discussion, and my wishful thinking,
> and we never actually followed through.

Maciej Rozycki still cares about i486 type hardware, and he was
asking for it to be kept around in the thread following [1]

I think the best suggestion at the time was to make cmpxchg8b
a compile-time feature and I had expected Maciej to follow up with
a patch for that, but this never happend, and nobody sent a patch
to remove support 486 and the early 586 clones either.

I saw recently that there are still distros that advertise 486
support on modern kernels: Tiny Core Linux and Damn Small
Linux. Both ship with a 486 SMP kernel but fail to boot
on qemu unless an APIC is enabled (DSL also requires i686 or
higher to run userspace).

       Arnd

[1] https://lore.kernel.org/all/20220815071332.627393-9-yuzhao@google.com/