From: "H. Hartzer" <h@hartzer.sh>
Subject: Incorrect Meltdown/Spectre mitigation exceptions for AMD64
To: <tech@openbsd.org>
Date: Sat, 19 Jul 2025 00:27:26 +0000

Hi tech@,

I have a Intel Atom S1260 system that I finally installed OpenBSD on. It
boots and shows that Meltdown issues are noticed, and I assume that
Meltdown mitigations are turned on.

This is not useful, however, because this processor has in-order
execution and is not subject to these vulnerabilities.

Poking around replacemeltdown() in sys/arch/amd64/amd64/cpu.c, what's
concerning is that if the comments are correct, the mitigation
exceptions exist for vulnerable processors.

Silvermont and Airmont are mentioned, which are out of order processors
that I believe suffer from this slew of issues.

I'm not sure how to identify processor families and models, but this
seems like it may be a fairly simple fix to speed up the older Atom
processors (Bonnell) and secure/slow down the newer Atoms listed.

Does sysctl machdep.cpuid provide the family/model info?

Thanks!

-Henrich