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

To: tech-kern%netbsd.org@localhost, port-amd64%netbsd.org@localhost
Subject: Why NetBSD x86's bus_space_barrier does not use [sml]fence?
From: Shoichi Yamaguchi <s.ymgch228%gmail.com@localhost>
Date: Fri, 29 Nov 2019 22:39:09 +0900

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).

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