linux-kernel - Re: [PATCH 1/5] x86, mm, pat: Set WT to PA4 slot of PAT MSR

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Message-ID: <CALCETrWxKFtM8FhnHQz--uaHYbiqShE1XLJxMCKN7Rs4SO14eQ@mail.gmail.com>
Date:	Thu, 4 Sep 2014 16:34:43 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Toshi Kani <toshi.kani@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, akpm@...uxfoundation.org,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 1/5] x86, mm, pat: Set WT to PA4 slot of PAT MSR

On Thu, Sep 4, 2014 at 4:19 PM, Henrique de Moraes Holschuh
<hmh@....eng.br> wrote:
> On Thu, 04 Sep 2014, H. Peter Anvin wrote:
>> On 09/04/2014 01:11 PM, Henrique de Moraes Holschuh wrote:
>> > I am worried of uncharted territory, here.  I'd actually advocate for not
>> > enabling the upper four PAT entries on IA-32 at all, unless Windows 9X / XP
>> > is using them as well.  Is this a real concern, or am I being overly
>> > cautious?
>>
>> It is extremely unlikely that we'd have PAT issues in 32-bit mode and
>> not in 64-bit mode on the same CPU.
>
> Sure, but is it really a good idea to enable this on the *old* non-64-bit
> capable processors (note: I don't mean x86-64 processors operating in 32-bit
> mode) ?
>
>> As far as I know, the current blacklist rule is very conservative due to
>> lack of testing more than anything else.
>
> I was told that much in 2009 when I asked why cpuid 0x6d8 was blacklisted
> from using PAT :-)

At the very least, anyone who plugs an NV-DIMM into a 32-bit machine
is nuts, and not just because I'd be somewhat amazed if it even
physically fits into the slot. :)

--Andy

>
> --
>   "One disk to rule them all, One disk to find them. One disk to bring
>   them all and in the darkness grind them. In the Land of Redmond
>   where the shadows lie." -- The Silicon Valley Tarot
>   Henrique Holschuh



-- 
Andy Lutomirski
AMA Capital Management, LLC
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