On Mon, Feb 27, 2023 at 05:38:49PM +0100, Neil Armstrong wrote:

On 27/02/2023 17:15, Arnd Bergmann wrote:

quoted

On Mon, Feb 27, 2023, at 16:51, Dmitry Rokosov wrote:

quoted

On Mon, Feb 27, 2023 at 03:58:50PM +0100, Arnd Bergmann wrote:

quoted

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!

Looks like Thomas has already prepared a basic patch series for buildroot,
but maintainers declined it.

https://lore.kernel.org/all/20220730194331.GA2515056@scaer/ (local)

-- 
Thank you,
Dmitry

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