Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 21 Oct 2022 12:12:27 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Rapoport <rppt@kernel.org>, Tejun Heo <tj@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        page-reclaim@google.com, Brian Geffon <bgeffon@google.com>,
        Jan Alexander Steffens <heftig@archlinux.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Steven Barrett <steven@liquorix.net>,
        Suleiman Souhlal <suleiman@google.com>,
        Daniel Byrne <djbyrne@mtu.edu>,
        Donald Carr <d@chaos-reins.com>,
        Holger =?iso-8859-1?Q?Hoffst=E4tte?= 
        <holger@applied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>,
        Shuang Zhai <szhai2@cs.rochester.edu>,
        Sofia Trinh <sofia.trinh@edi.works>,
        Vaibhav Jain <vaibhav@linux.ibm.com>
Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks
Message-ID: <Y1Jwi9xS8ZCdbIUY@hirez.programming.kicks-ass.net>
References: <20220815071332.627393-1-yuzhao@google.com>
 <20220815071332.627393-9-yuzhao@google.com>
 <Y0go8wWtdcyH1+Ch@hirez.programming.kicks-ass.net>
 <CAOUHufa9+FTO3Pv-5jC-e3S5goPsUGu-5KcPVHa4bWb0X+d2ug@mail.gmail.com>
 <CAHk-=wj1rc2t5noMtVOgu8XXeTM4KiggEub9PdcexxeQrYPZvA@mail.gmail.com>
 <Y1FXpHdyvXjrjbLw@hirez.programming.kicks-ass.net>
 <CAHk-=whQchubuDpRGFabhmcZuzdt13OOF8wznXb+Dbi3GzBQhQ@mail.gmail.com>
 <Y1GZjPO+szk7X0wP@hirez.programming.kicks-ass.net>
 <CAHk-=wikUaRM5H_y1Bc+QyvGi40dKDL8fnCTyz7ECbwK7aHNPQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wikUaRM5H_y1Bc+QyvGi40dKDL8fnCTyz7ECbwK7aHNPQ@mail.gmail.com>
Precedence: bulk

On Thu, Oct 20, 2022 at 07:10:46PM -0700, Linus Torvalds wrote:

> 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.

Would love that; but as pointed out, there's still a few stragglers out
there. OTOH, they *could* use one of the LTS kernels.