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

[Neil Armstrong]

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