Why NetBSD x86's bus_space_barrier does not use [sml]fence?

[Shoichi Yamaguchi <s.ymgch228%gmail.com@localhost>]

Hi, all

FreeBSD and OpenBSD use memory fences(mfence, sfence, lfence) to
implement bus_space_barrier().
On the other hand, NetBSD does not use them.

Do you know the background about current implementation?

I found an issue caused by this implementation difference while porting
ixl(4).