The x32 system call ABI

There is also:

(3) Keep the source level API 32-bit, but have glibc do the zero-extension into the true 64-bit API before calling the kernel.

The main problem with that is, of course, ioctl (the same compat ioctl problem the kernel already has). So, how about this:

(4) Same as (3) but add a new x32_ioctl 64-bit syscall which calls into the compat ioctl engine the kernel already has.