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

[Gareth Poole]

As someone who still regularly uses hardware from this era, and often 
runs Linux on it, this would definitely be a blow to which machines I 
can actively use. Linux support is a big part of how I use these 
machines, since DOS and Windows 95 really can’t keep up with modern 
networking standards.

I would be very disappointed, and impacted, if Linux dropped 486 support.

On 10/20/22 23:10, Linus Torvalds wrote:
> On Thu, Oct 20, 2022 at 11:55 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Thu, Oct 20, 2022 at 10:35:28AM -0700, Linus Torvalds wrote:
> > > That said, I reacted to that cmpxchg loop:
> > >
> > >          } while (cmpxchg64(&pmdp->pmd, old.pmd, 0ULL) != old.pmd);
> > >
> > > is this series just so old (but rebased) that it doesn't use "try_cmpxchg64()"?
> > Yep -- it's *that* old :-/ You're not in fact the first to point that
> > out.
> >
> > I'll make time tomorrow to fix it up and respin and send out. This is as
> > good a time as any to get rid of carrying these patches myself.
> Hmm. Thinking some more about it, even if you do it as a
> try_cmpxchg64() loop, it ends up being duplicated twice.
>
> Maybe we should just bite the bullet, and say that we only support
> x86-32 with 'cmpxchg8b' (ie Pentium and later).
>
> Get rid of all the "emulate 64-bit atomics with cli/sti, knowing that
> nobody has SMP on those CPU's anyway", and implement a generic x86-32
> xchg() setup using that try_cmpxchg64 loop.
>
> I think most (all?) distros already enable X86_PAE anyway, which makes
> that X86_CMPXCHG64 be part of the base requirement.
>
> Not that I'm convinced most distros even do 32-bit development anyway
> these days.
>
> (Of course, if we require X86_CMPXCHG64, we'll also hit some of the
> odd clone CPU's that actually *do* support the instruction, but do not
> report it in cpuid due to an odd old Windows NT bug. IOW, things like
> the Cyrix and Transmeta CPU's did support the instruction, but had the
> CX8 bit clear because otherwise NT wouldn't boot. We may or may not
> get those cases right, but I doubt anybody really has any of those old
> CPUs).
>
> We got rid of i386 support back in 2012. Maybe it's time to get rid of
> i486 support in 2022?
>
> That way we could finally get rid of CONFIG_MATH_EMULATION too.
>
>                 Linus