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

[Dmitry Rokosov]

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