On Linux and Windows, I'm used to the situation that I require a 64-bit kernel to have a system with multiarch/WoW where I can run 32-bit and 64-bit software side-by-side.
And then, years ago it blew my mind when someone showed me that MacOS 10.6 Snow Leopard could run 64-bit applications with the kernel in 32-bit mode. This may be largely forgotten now because it was a one-time technology transition. With the hardware ahead of the curve in the mobile space, as far as I know this was never needed in the move to 64-bit for iOS and Android.
My question: What would it take to get the same capability in a 32-bit Linux kernel (i386 or armhf)?
I understand that this probably isn't trivial. If it was, Microsoft could have put the feature into Windows XP 32-bit. What are the general requirements though? Has there ever been a proposed patch or proof-of-concept?
In the embedded world I think this would be especially helpful, as 64-bit support can lag behind for a long time in device drivers.