linux-kernel - Re: [PATCH v1 0/3] Meson A1 32-bit support

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Message-ID: <20230227163716.fxlczbqol2csumpa@CAB-WSD-L081021>
Date:   Mon, 27 Feb 2023 19:37:16 +0300
From:   Dmitry Rokosov <ddrokosov@...rdevices.ru>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Neil Armstrong <neil.armstrong@...aro.org>,
        Alexey Romanov <avromanov@...rdevices.ru>,
        Rob Herring <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>,
        Kevin Hilman <khilman@...libre.com>, <jbrunet@...libre.com>,
        <martin.blumenstingl@...glemail.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-amlogic@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kernel@...rdevices.ru>
Subject: Re: [PATCH v1 0/3] Meson A1 32-bit support

On Mon, Feb 27, 2023 at 05:15:58PM +0100, 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.

Thank you very for the detailed clarification! It's strong arguments.

-- 
Thank you,
Dmitry