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Date:   Mon, 27 Feb 2023 17:15:58 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Dmitry Rokosov" <ddrokosov@sberdevices.ru>
Cc:     "Neil Armstrong" <neil.armstrong@linaro.org>,
        "Alexey Romanov" <avromanov@sberdevices.ru>,
        "Rob Herring" <robh+dt@kernel.org>,
        krzysztof.kozlowski+dt@linaro.org,
        "Kevin Hilman" <khilman@baylibre.com>, jbrunet@baylibre.com,
        martin.blumenstingl@googlemail.com,
        "Linus Walleij" <linus.walleij@linaro.org>,
        devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-amlogic@lists.infradead.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru
Subject: Re: [PATCH v1 0/3] Meson A1 32-bit support
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On Mon, Feb 27, 2023, at 16:51, Dmitry Rokosov wrote:
> On Mon, Feb 27, 2023 at 03:58:50PM +0100, Arnd Bergmann wrote:
>> 
>> I would argue that is a problem with buildroot, and using a 32-bit
>> kernel is not something we should encourage over fixing buildroot
>> to do it right, or building the kernel separately from the rootfs.
>> 
>> We do allow building support for a couple of ARMv8 SoCs in 32-bit
>> mode, but that is usually because they ship with a 32-bit bootrom
>> and cannot actually run a 64-bit kernel.
>
> To be honest, I didn't know about this principle. It looks like a very
> rational approach "start from max supported bitness".
> Based on overall maintainers opinion, we have to prepare a patch for
> buildroot to support compat mode :)

That would be great, thanks a lot!

For what it's worth, the main arguments in favor of running a 64-bit
kernel with compat user space over a 32-bit kernel are support for:

- larger RAM sizes without highmem (most 32-bit kernels only
  support 768MB of lowmem, and highmem sucks)
- larger virtual address space (4GB vs 3GB or less)
- CPU specific errata workarounds (arch/arm/ only has those for 32-bit cpus)
- mitigations for common attacks such as spectre
- security hardening that depends on larger address space
  (KASLR, BTI, ptrauth, PAN, ...)
- emulating instructions that were removed in Armv8 (setend, swp, ...)

Most of these don't apply in userspace, so the incentive to
run smaller 32-bit userland on systems with less than 1GB of
RAM usually outweighs the benefits of 64-bit userspace.

      Arnd