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

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Date:	Mon, 3 Nov 2014 18:14:37 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Toshi Kani <toshi.kani@...com>
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, 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?

> +		 *
> +		 *  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 ....

> +		 *      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,

	tglx
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