linux-kernel - Re: [PATCH v3 0/2] Forcibly enable some MISC

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Date:	Tue, 9 Aug 2011 13:50:50 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andy Lutomirski <luto@....EDU>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	Fenghua Yu <fenghua.yu@...el.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v3 0/2] Forcibly enable some MISC_ENABLE features on
 Intel

On Tue,  9 Aug 2011 08:41:04 -0400
Andy Lutomirski <luto@....EDU> wrote:

> Intel allows BIOS or the OS to enable or disable some CPU fueatures via
> IA32_MISC_ENABLE.  I have machines that don't enable fast strings or
> monitor/mwait in BIOS, so do it on bootup instead.

The question is - why. 

> The Intel SDM volume 3, appendix B.1 says that the OS should not touch
> the monitor enable bit if SSE3 is not present, which presumably means
> that the OS may touch that bit if SSE3 is present.  In any case, these
> patches seem to work.

That's a big "presumes"

For example back in Pentium days the BIOS sometimes turned off a string
fast path because it was subtly not reliable on certain parts and could
corrupt.

It's possible your BIOS vendor has a good reason for doing this. It's
also possible its a bug, or a left over perhaps from some pre-release
processor or something.

I think a bit more research is probably appropriate before processor
configuration hacks go in that 'seem to work'.

Alan
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