64 bit kernel with 32 bit userland

It's possible to run a 64-bit kernel on a system where everything else is 32-bits.

When installing from deb packages, the OS needs to have the arm64 architecture enabled. However, this will cause apt to try to read 64-bit packages from any connected apt repos. The solution is to set "arch=armhf" for all repos that apt knows about (under /etc/apt/)

For Raspberry Pi OS, the following commands may be used:

# if installing 64-bit kernel on 32-bit system:
dpkg --add-architecture arm64
sed -i 's|^deb http|deb [ arch=armhf ] http|g' /etc/apt/sources.list
sed -i 's|^deb http|deb [ arch=armhf ] http|g' /etc/apt/sources.list.d/raspi.list