linux-kernel - Re: [PATCH v4 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR

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Date:	Mon, 03 Nov 2014 10:47:59 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	hpa@...or.com, mingo@...hat.com, akpm@...ux-foundation.org,
	arnd@...db.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	jgross@...e.com, stefan.bader@...onical.com, luto@...capital.net,
	hmh@....eng.br, yigal@...xistor.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v4 1/7] x86, mm, pat: Set WT to PA7 slot of PAT MSR

On Mon, 2014-11-03 at 18:14 +0100, Thomas Gleixner wrote:
> On Mon, 27 Oct 2014, Toshi Kani wrote:
> > +	} else {
> > +		/*
> > +		 * PAT full support. WT is set to slot 7, which minimizes
> > +		 * the risk of using the PAT bit as slot 3 is UC and is
> > +		 * currently unused. Slot 4 should remain as reserved.
> 
> This comment makes no sense. What minimizes which risk and what has
> this to do with slot 3 and slot 4?

This is for precaution.  Since the patch enables the PAT bit the first
time, it was suggested that we keep slot 4 reserved and set it to WB.
The PAT bit still has no effect to slot 0/1/2 (WB/WC/UC-) after this
patch.  Slot 7 is the safest slot since slot 3 (UC) is unused today.

https://lkml.org/lkml/2014/9/4/691
https://lkml.org/lkml/2014/9/5/394

> > +		 *
> > +		 *  PTE encoding used in Linux:
> > +		 *      PAT
> > +		 *      |PCD
> > +		 *      ||PWT  PAT
> > +		 *      |||    slot
> > +		 *      000    0    WB : _PAGE_CACHE_MODE_WB
> > +		 *      001    1    WC : _PAGE_CACHE_MODE_WC
> > +		 *      010    2    UC-: _PAGE_CACHE_MODE_UC_MINUS
> > +		 *      011    3    UC : _PAGE_CACHE_MODE_UC
> > +		 *      100    4    <reserved>
> > +		 *      101    5    <reserved>
> > +		 *      110    6    <reserved>
> 
> Well, they are still mapped to WB/WC/UC_MINUS ....

Right, the reserved slots are also initialized with their safe values.
However, the macros _PAGE_CACHE_MODE_XXX only refer to the slots
specified above.

> > +		 *      111    7    WT : _PAGE_CACHE_MODE_WT
> > +		 */
> > +		pat = PAT(0, WB) | PAT(1, WC) | PAT(2, UC_MINUS) | PAT(3, UC) |
> > +		      PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, WT);
> > +	}
> 
> Thanks,

Thanks for the review!
-Toshi

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