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

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Message-ID: <CAHk-=wjCiEk-kc-vOug2GKJdhHKce3vWALbqjybLPcKLHNmEbQ@mail.gmail.com>
Date: Tue, 6 May 2025 10:11:04 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: "Maciej W. Rozycki" <macro@...am.me.uk>, Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, 
	"Ahmed S . Darwish" <darwi@...utronix.de>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, John Ogness <john.ogness@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [RFC PATCH 0/15] x86: Remove support for TSC-less and CX8-less CPUs

On Tue, 6 May 2025 at 09:44, H. Peter Anvin <hpa@...or.com> wrote:
>
> a. Someone would have to take it on;
> b. It will need continuous testing;
> c. That someone would *also* have to go through the additional effort of keeping the mainline code clean for the maintainers of the modern hardware.

I think the main issue is "when problems happen, people who
*shouldn't* have to care get reports".

I really think that the way forward is basically what we did for ia64:
get rid of i486 support in mainline, and people who care about i486
can maintain a smallish patch that basically keeps it alive for them.

Because I suspect that the "patch to keep it working in practice" is
likely going to remain pretty small: it's the silly cmpxchg helper
wrappers, it's disabling ARCH_USE_CMPXCHG_LOCKREF, and probably a few
X86_FEATURE_CX8 tests.

And it probably (a) works fine and (b) won't be code that changs very
much upstream, so maintaining it outside the main tree is likely not a
lot of work.

But because it's outside of the main tree, it won't cause pointless
noise from 0day bots etc, and won't affect people who care about
modern machines. And it can do various hacky things because the patch
would *only* be used by people who actually run on an i486-class
machine.

(Ok, if you actually care about the i486SX, the patch will be much
bigger, because it will have that whole FPU emulation code in it)

             Linus