Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks

[Arnd Bergmann]

On Sun, Oct 23, 2022, at 20:35, Linus Torvalds wrote:
>
> Honestly, I wouldn't mind upgrading the minimum requirements to at
> least M586TSC - leaving some of those early "fake Pentium" clones
> behind too. Because 'rdtsc' is probably an even worse issue than
> CMPXCHG8B.

Kconfig treats X86_CMPXCHG64 as a strict subset of X86_TSC (except
when enabling X86_PAE, which relies on cx8), so requiring both
sounds like a good idea.

>From the Kconfig history, I see you initially only enabled
cx8 unconditionally for a couple of CPUs in 982d007a6eec ("x86:
Optimize cmpxchg64() at build-time some more"), and Matthew
Whitehead extended that list in f960cfd12650 ("x86/Kconfig:
Add missing i586-class CPUs to the X86_CMPXCHG64 Kconfig group").

There are still a handful of CPUs that according to [1] 
claim cx8 support that we leave disabled, specifically the
Kconfig symbols for MWINCHIP3D, MCRUSOE, MEFFICEON, MCYRIXIII,
MVIAC3_2 and MVIAC7 should have both tsc and cx8, while the
older MWINCHIPC6 and a small subset of M586 (Cyrix 6x86mx, C-II
and AMD K5) apparently have cx8 but not tsc.

Would you drop support for the 686-class chips that currently
don't use cmpxchg8b, or just remove CONFIG_X86_CMPXCHG64 and
assume they work after all?

       Arnd

[1] https://reactos.org/wiki/Supported_Hardware/CPU