linux-kernel - Re: [RFC PATCH 0/15] x86: Remove support for TSC-less and CX8-less CPUs

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Message-Id: <dd29df0c-0b4f-44e6-b71b-2a358ea76fb4@app.fastmail.com>
Date: Thu, 24 Apr 2025 23:27:42 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ingo Molnar" <mingo@...nel.org>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc: "kernel test robot" <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
 "kernel test robot" <lkp@...el.com>, linux-kernel@...r.kernel.org,
 "John Stultz" <jstultz@...gle.com>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Stephen Boyd" <sboyd@...nel.org>, "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, "Peter Zijlstra" <peterz@...radead.org>
Subject: Re: [RFC PATCH 0/15] x86: Remove support for TSC-less and CX8-less CPUs

On Thu, Apr 24, 2025, at 19:54, Ingo Molnar wrote:
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>> I really get the feeling that it's time to leave i486 support behind. 
>> There's zero real reason for anybody to waste one second of 
>> development effort on this kind of issue.
>
> Fully agreed!
>
> And to turn this idea into code, here's a very raw RFC series that 
> starts removing non-TSC 586 and 486 code and related support code from 
> the x86 architecture, with the goal to make TSC and CX8 (CMPXCHG8B) 
> support unconditionally available:

Yes, makes sense. I had considered doing something like this in
my cleanup for large machines, but decided to keep stop there
because I know that there are users that love their museum pieces.

For embedded systems, I'm quite sure that the AMD Élan, SIS 55x,
RDC321x and Vortex86SX have ended their useful life. You may
still be able to buy Vortex86SX machines and they are probably
still running somewhere, but the entire point of those machines
is to run old software. DM&P provides patches for linux-2.6.29
and Windows CE 6.0 for the SX chips.

The desktop Socket 7 clones without CX8/TSC all got discontinued
over 25 years ago, and they were rare even then.

>       x86/platform: Remove CONFIG_X86_RDC321X support
>       arch/x86, gpio: Remove GPIO_RDC321X support
>       arch/x86, watchdog: Remove the RDC321X_WDT watchdog driver
>       arch/x86, mfd: Remove MFD_RDC321X support
>       x86/reboot: Remove the RDC321X reboot quirk

I'm not sure about the RDC321X bits. Obviously the original
321x/861x/vortex86sx chips are obsolete and can be removed,
but the product line is still actively developed by RDC and 
DM&P, and I suspect that some of the drivers are still used
on 586tsc-class (vortex86dx, vortex86mx) and 686-class
(vortex86dx3, vortex86ex) SoCs that do run modern kernels and
get updates.

>       x86/cpu: Remove CPU_SUP_UMC_32 support
>       x86/cpu: Remove TSC-less CONFIG_M586 support

I think Winchip6 (486-class, no tsc, no cx8) and Winchip3D
(486-class, with tsc but no cx8) need to go as well then.

At this point, maybe we can consider removing
CONFIG_X86_GENERIC and just always build kernels that work
across a wide set of CPUs: Only CMOV and PAE still require a
CPU with the hardware support, and X86_L1_CACHE_SHIFT needs to
be at least 6 (64 byte) for compatibility, but everything
else should just be a tuning option.

      Arnd