The same way those 32-bit libraries call the kernel now - they marshal the call to 64-bit land. A lot of the user space is compiled separately for performance a

The same way those 32-bit libraries call the kernel now - they marshal the call to 64-bit land. A lot of the user space is compiled separately for performance and to maintain ABI compatibility so this would have to be taken up by a third party. I would expect to see a slowdown but it depends on the exact nature of the changes.