linux-kernel - Re: [PATCH -v3 0/15] x86: Remove support for TSC-less and CX8-less CPUs

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Message-ID: <20251214133925.1e80f851@pumpkin>
Date: Sun, 14 Dec 2025 13:39:25 +0000
From: David Laight <david.laight.linux@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: 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>, "H . Peter
 Anvin" <hpa@...or.com>, John Ogness <john.ogness@...utronix.de>, Linus
 Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra
 <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH -v3 0/15] x86: Remove support for TSC-less and CX8-less
 CPUs

On Sun, 14 Dec 2025 09:46:48 +0100
Ingo Molnar <mingo@...nel.org> wrote:

>...
> 
> Original -v1 announcement:
> 
> In the x86 architecture we have various complicated hardware emulation
> facilities on x86-32 to support ancient 32-bit CPUs that very very few
> people are using with modern kernels. This compatibility glue is sometimes
> even causing problems that people spend time to resolve, which time could
> be spent on other things.
> 
> As Linus recently remarked:
> 
>  > 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.  
> 
> This series increases minimum kernel support features to include TSC and
> CX8 (CMPXCHG8B) hardware support, which removes 486 (and derivatives) support
> and early-586 (and derivatives) support.

Two questions:

1) What happens if you try to boot the kernel on an old system?
   Is there an early check that makes it fail 'moderately gracefully'?

2) Is it worth also dropping support for cpu that don't support CMOV.
   I think that is 'Pentium Pro' and 'K7' rather than 'Pentium' and 'K5'.
   That simplifies some asm code.
   'masked' user access would really need it (and a guard page).
   The 'long nop' get added at the same time.
   It is also (almost certainly) new enough that the 'bit scan' instructions
   are guaranteed to leave the the destination unchanged if the source is
   zero (dropping just 486 may be enough for that).

Moving the minimum cpu to P-Pro now will probably save another similar
change in the near future.

	David