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

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Message-ID: <5409D99C.10305@zytor.com>
Date:	Fri, 05 Sep 2014 08:41:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	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 09/05/2014 08:22 AM, Toshi Kani wrote:
> On Fri, 2014-09-05 at 08:07 -0700, H. Peter Anvin wrote:
>> On 09/05/2014 07:00 AM, Toshi Kani wrote:
>>>
>>> That's a fine idea, but as Ingo also suggested, I am going to disable
>>> this feature on all Pentium 4 models.  That should give us a safety
>>> margin.  Using slot 4 has a benefit that it keeps the PAT setup
>>> consistent with Xen.      
>>>
>>
>> Slot 4 is also the maximally problematic one, because it is the one that
>> might be incorrectly invoked for the page tables themselves.
> 
> Good point.  I wonder if Xen folks feel strongly about keeping the PAT
> setup consistent with the kernel.  If not, we may choose to use slot 6
> (or 7).
> 

Who cares what the Xen folks "feel strongly about"?  If strong feelings
were a design criterion Xen support would have been pulled from the
kernel a long, long time ago.

The important thing is how to design for the situation that we currently
have to live with.

	-hpa


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