Re: Architecture baseline for Forky

From	Peter Green <plugwash@debian.org>
Newsgroups	linux.debian.ports.arm, linux.debian.kernel
Subject	Re: Architecture baseline for Forky
Date	2025-10-30 10:10 +0100
Message-ID	<LLwPn-96Ea-1@gated-at.bofh.it> (permalink)
References	(6 earlier) <LKD6y-8sUr-3@gated-at.bofh.it> <LL9g5-8Q1a-1@gated-at.bofh.it> <LLtRw-94j2-3@gated-at.bofh.it> <LLtRw-94j2-1@gated-at.bofh.it> <LLwPn-96Ea-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Cross-posted to 2 groups.

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On 30/10/2025 08:28, Arnd Bergmann wrote:
> On Thu, Oct 30, 2025, at 06:39, Peter Green wrote:
>> On 29/10/2025 13:47, Arnd Bergmann wrote:
>>
>>   > 5. armv6k+vfpv3d16: Most ARMv6/v7 machines, including Raspberry
>>   >     Pi zero/1 but not OMAP2 (Nokia N800/N810). Loses THUMB2 support
>>   >     and v7/v8 CPU barriers among other minor differences.
>>
>> As maintainer of raspbian, I can say the memory barrier issue is
>> one of the bigger thorns in our side and is the reason we continue
>> to configure gcc for armv6 rather than armv6k.
>>
>> For the uninitiated, arm added memory barrier instructions in armv6k,
>> as instructions on the system coprocessor (aka CP15). When targetting
>> armv6k, compilers use these instructions to implement
>> acquire/release/seqcst atomics.
>>
>> With armv7-a, arm deprecated the CP15 barriers and introduced the
>> dmb instruction.
>>
>> The problem is that arm64 kernels by default trap the armv6k memory
>> barriers into the kernel and emulate them. Asside from being slow
>> this can also cause hangs if the optimiser moves a barrier inside
>> a load-exclusive/store-exclusive loop.
> Right, this is very unfortunate, and I suspect that this is something
> we should change in both the arm64 kernel and in the compiler. If
> you can point me to specific userspace code that has this problem,
> I can probably come up with a kernel patch to work around it,
> at least if that userspace code isn't obviously wrong.
https://github.com/rust-lang/rust/issues/53670

Thread

Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-26 13:40 +0100
  Re: Architecture baseline for Forky John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> - 2025-10-26 14:20 +0100
    Re: Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-27 08:40 +0100
  Re: Architecture baseline for Forky Tony Rodriguez <unixpro1970@gmail.com> - 2025-10-26 19:40 +0100
    Re: Architecture baseline for Forky John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> - 2025-10-26 20:30 +0100
    Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-27 12:50 +0100
      Re: Architecture baseline for Forky Martin <debacle@debian.org> - 2025-10-27 18:10 +0100
      Re: Architecture baseline for Forky "Arnd Bergmann" <arnd@arndb.de> - 2025-10-27 22:40 +0100
        Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-29 09:00 +0100
          Re: Architecture baseline for Forky Peter Green <plugwash@debian.org> - 2025-10-30 07:00 +0100
            Re: Architecture baseline for Forky Peter Green <plugwash@debian.org> - 2025-10-30 10:10 +0100
              Re: Architecture baseline for Forky "Arnd Bergmann" <arnd@arndb.de> - 2025-10-30 10:10 +0100
          Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-30 15:30 +0100
    Re: Architecture baseline for Forky Rob Landley <rob@landley.net> - 2025-10-27 21:40 +0100
  Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-27 11:20 +0100
  Re: Architecture baseline for Forky Colin Watson <cjwatson@debian.org> - 2025-10-27 19:30 +0100
    Re: Architecture baseline for Forky "Nikos Tsipinakis" <nikos@tsipinakis.com> - 2025-10-30 14:40 +0100
  Re: Architecture baseline for Forky Milan Kupcevic <milan@debian.org> - 2025-10-27 22:40 +0100
    Re: Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-27 23:40 +0100
      Re: Architecture baseline for Forky Marc Haber <mh+debian-release@zugschlus.de> - 2025-10-28 10:10 +0100
    Planned obsolescence ? (was: Re: Architecture baseline for Forky) Romain Dolbeau <romain@dolbeau.org> - 2025-10-28 10:00 +0100
      Re: Planned obsolescence ? (OpenBSD) Paul Tagliamonte <paultag@debian.org> - 2025-11-02 23:30 +0100
      Re: Planned obsolescence ? (*BSD, Rust) David Starner <prosfilaes@gmail.com> - 2025-11-04 00:20 +0100
  Re: Architecture baseline for Forky Rob Landley <rob@landley.net> - 2025-12-05 00:20 +0100
  R: Re: Architecture baseline for Forky Federico Vaga <federico.vaga@vaga.pv.it> - 2026-09-07 09:20 +0200

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