64 Bit Kernels With 32 Bit Userspace
As 64-bit processors are becoming more common in embedded systems, there is significant need for software to run native 64bit. However, there are challenges involved first in ensuring that applications are correctly ported which is a time consuming task, and in some cases it may not make sense to port a legacy application to 64-bit. However it is essential for a complete solution.
New SBCs and SOMs are increasingly being designed using the 64bit ARM (aarch64) CPU architecture. Examples include the RaspberryPI3 (cortext-a53), RaspberryPI4 (cortex-a72), and iMX8 based SOMs – these all use the armv8+ architecture. The board support packages for these platforms all support native 64bit and are quite stable. However, some applications stacks may still require 32bit mode support for various reasons. Additionally, 64-bit applications typically use more DRAM and may perform slower than the same application compiled in 32-bit mode.
Therefore, we have several options for running 32bit applications on 64bit platforms:
- Use full 32bit mode, which would limit the use of 64bit ISA completely
- Run kernel in 64bit mode but let whole userspace run as 32bit bit
- Run kernel in 64bit mode with 64bit userspace and additionally support 32bit runtime
These options come with their own set of tradeoffs.
Doing everything in 32bit mode (#1) gives us backward compatibility, but then we can’t use 64bit mode ISA and we are limited to a 4GB virtual memory space.
The second option lets one use the BSP defaults to run the kernel in 64bit mode but keeps userspace 32bit. This is the path of least resistance for using existing userspace software without any significant porting efforts.
The third option offers a mix of 32bit and 64bit userspace, which comes with added duplication of runtimes, increasing the static footprint of the firmware, which could be a design factor for a lot of embedded systems. Runtime DRAM requirements for applications is also higher, which might not be suitable for the embedded system at hand, as DRAM is a precious resource.
Enabling 32-bit images on 64-bit kernels in Yoe
Enable Yocto multilib configuration settings in
# multilib arm
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "armv7athf-neon-vfpv4"
These settings will instruct the build to enable building applications in 32bit
mode if desired,
DEFAULTTUNE is an important setting and it could be set to
same value as it was being used in pure 32bit build perhaps in older machine
Building 32bit-only Userspace Image
This should build a 64bit kernel and pure 32bit kiosk image with all multilib settings in place such that 64bit kernel can boot into 32bit userspace
Building 64bit+32bit mixed Userspace Image
Add needed 32bit applications to image via
IMAGE_INSTALL_append = " lib32-python3-core lib32-htop"
This will add 32bit version of htop application and 32bit python to the otherwise 64bit image
Now build the image
yoe_install_image /dev/sd<X> yoe-kiosk-image
X with the device letter ‘a’ ‘b’ ‘c’ .. Use
dmesg | tail to check
where SD card is mounted.
The Yoe distribution enjoys sound multilib support provided by the OpenEmbedded build system. Design choices can be made early in the software development process, and it is easy to generate different combinations of multi-libbed images and test the suitability of a given combination. Whatever your 32/64-bit needs are, Yoe gives you options to get there.