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
