Syscall API: thread creation

• A: pass desired stack pointer into the kernel and have it set it up

• rfork() doesn't have the arguments for this, so new syscall:

  pid_t __tfork(struct __tfork { void *tf_stack; ... } *params)

• annoying MD work in kernel (I hate you, hppa)

• still need MD ASM wrapper to call this: the child can't return from the stub that invokes the syscall trap