linux-kernel - Re: [PATCH 2/2] x86/mtrr: Refactor PAT initialization code

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Date:	Tue, 29 Mar 2016 17:34:08 -0700
From:	"Luis R. Rodriguez" <mcgrof@...nel.org>
To:	Toshi Kani <toshi.kani@....com>
Cc:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
	Paul Stewart <pstew@...omium.org>,
	Ingo Molnar <mingo@...nel.org>,
	Olof Johansson <olof@...om.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
	Jan Beulich <JBeulich@...e.com>,
	Stuart Hayes <stuart.w.hayes@...il.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>
Subject: Re: [PATCH 2/2] x86/mtrr: Refactor PAT initialization code

On Tue, Mar 29, 2016 at 6:07 PM, Toshi Kani <toshi.kani@....com> wrote:
> On Tue, 2016-03-29 at 16:43 -0700, Luis R. Rodriguez wrote:
>> I meant to ask about the case where the option the lets a user go in a
>> muck with BIOS settings to disable MTRR e xists and the user disables
>> MTRR. What would happen for fan control in such situations? I'd
>> imagine such cases allow for a system to exist with proper fan
>> control, and allow the kernel to boot without having to deal with the
>> pesky MTRRs at all, while PAT lives on, no?
>
> You mean user disables MTRRs from BIOS setup menu?

Yup!

> I am not a BIOS guy,
> but I do not think it offers such option when the code depends on it...

Darn, I'm pretty sure I've seen such option before... can't seem to
find such a toggle now.

>> When you say regular memory you mean everything else we see as RAM? I
>> was under the impression we'd only need MTRR for a special range of
>> memory, and its up to implementation how they are used. If you can use
>> MTRR to change the cache attribute for regular RAM and if this is
>> actually a requirement if the default MTRR is UC then one way or
>> another a BIOS seems to always require MTRR, either for UC setting for
>> fan control or WB for regular RAM, is that right?
>
> Right, in one way or the other, MTRRs set WB to RAM and UC to MMIO.  PAT is
> overwritten by MTRRs, so RAM must be set to WB.

I see... thanks....

  Luis