Message-ID: <33b58877-5167-c453-e686-1d10cdca66c0@linaro.org>
Date:   Mon, 27 Feb 2023 17:38:49 +0100
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From:   Neil Armstrong <neil.armstrong@linaro.org>
Reply-To: neil.armstrong@linaro.org
Subject: Re: [PATCH v1 0/3] Meson A1 32-bit support
To:     Arnd Bergmann <arnd@arndb.de>,
        Dmitry Rokosov <ddrokosov@sberdevices.ru>
Cc:     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
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On 27/02/2023 17:15, Arnd Bergmann wrote:
> 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.

Thanks for the details!

Neil

> 
>        Arnd