To get clean state we should probably clear it as well. That's probably a (small) bug that we don't clear it now.
We could perhaps assert that neither KERNFPU nor KERNFPU_THR is set here instead of clearing. No user process calling thr_new() should have active kernel FPU state here. cpu_fork() doesn't currently bother clearing this flag (so I didn't add it), but I would like these two code blocks to be as identical as possible between cpu_fork() and cpu_copy_thread(). (It's somewhat temping to see if I could come up with some kind of 'copy_pcb' function for the shared logic between the two functions.)
This comment seems a bit stale as it doesn't create a stack at all. For exec_setregs where I think this was copied from, we bzero the trap frame before setting registers. I wonder if for the trapframe cpu_thread_copy() shouldn't be more like exec and rely on cpu_set_upcall() instead.
I tried doing a bzero here and it didn't work out well. cpu_set_upcall() below doesn't set tf_ss (though it sets all the others) which blew up. The 32-bit cpu_set_upcall() doesn't set any seg regs at all, and linux_set_upcall_kse() assumes a copy instead of clear. I will make the comment more accurate though.