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From: "Luis R. Rodriguez" <mcgrof@kernel.org>
Date: Tue, 29 Mar 2016 17:34:08 -0700
Message-ID: <CAB=NE6XEMdwpafGt5csNGeDe3Y9T8Sw0w27+ryptrUHWYs-t2w@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/mtrr: Refactor PAT initialization code
To: Toshi Kani <toshi.kani@hpe.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
        Paul Stewart <pstew@chromium.org>, Ingo Molnar <mingo@kernel.org>,
        Olof Johansson <olof@lixom.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Juergen Gross <jgross@suse.com>,
        Jan Beulich <JBeulich@suse.com>,
        Stuart Hayes <stuart.w.hayes@gmail.com>,
        Yinghai Lu <yinghai@kernel.org>, Prarit Bhargava <prarit@redhat.com>
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On Tue, Mar 29, 2016 at 6:07 PM, Toshi Kani <toshi.kani@hpe.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