what is opaque parameter in the init function of linux kernel module

Question

As the title asks, what is opaque parameter in the init function of linux kernel module?

I've seen this in kvm_init function, and failed to see the reference of it.

Answer 1

Opaque means that this parameter is deliberately unexplained, because no guarantee is given about its format.

If the parameter were documented, someone would write a program manipulating it, which would fail in curious and interesting ways when some internal design changes.

It is questionable whether this is actually good programming practice.