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

From: Dmitry Rokosov <ddrokosov@sberdevices.ru>
To: Arnd Bergmann <arnd@arndb.de>
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
Date: Mon, 27 Feb 2023 19:37:16 +0300	[thread overview]
Message-ID: <20230227163716.fxlczbqol2csumpa@CAB-WSD-L081021> (raw)
In-Reply-To: <a5fa8b23-4ec8-475f-be5e-538b53d6f82d@app.fastmail.com>

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

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