linux-kernel - Re: [PATCH v2 1/6] x86, mm, pat: Set WT to PA4 slot of PAT MSR

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Date:	Fri, 12 Sep 2014 15:33:45 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Toshi Kani <toshi.kani@...com>
Cc:	hpa@...or.com, tglx@...utronix.de, 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
Subject: Re: [PATCH v2 1/6] x86, mm, pat: Set WT to PA4 slot of PAT MSR

> -	/* Set PWT to Write-Combining. All other bits stay the same */
> -	/*
> -	 * PTE encoding used in Linux:
> -	 *      PAT
> -	 *      |PCD
> -	 *      ||PWT
> -	 *      |||
> -	 *      000 WB		_PAGE_CACHE_WB
> -	 *      001 WC		_PAGE_CACHE_WC
> -	 *      010 UC-		_PAGE_CACHE_UC_MINUS
> -	 *      011 UC		_PAGE_CACHE_UC


I think having this nice picture would be beneficial to folks
who want to understand it. And now you can of course expand it with
the slot 7 usage.

> -	 * PAT bit unused
> -	 */
> -	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, UC);
> +	if ((c->x86_vendor == X86_VENDOR_INTEL) &&
> +	    (((c->x86 == 0x6) && (c->x86_model <= 0xd)) ||
> +	     ((c->x86 == 0xf) && (c->x86_model <= 0x6)))) {
> +		/*
> +		 * Intel Pentium 2, 3, M, and 4 are affected by PAT errata,
> +		 * which makes the upper four entries unusable.  We do not
> +		 * use the upper four entries for all the affected processor
> +		 * families for safe.
> +		 *
> +		 * PAT 0:WB, 1:WC, 2:UC-, 3:UC, 4-7:unusable
> +		 *
> +		 * NOTE: When WT or WP is used, it is redirected to UC- per
> +		 * the default setup in  __cachemode2pte_tbl[].
> +		 */
> +		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, UC);
> +	} else {
> +		/*
> +		 * WT is set to slot 7, which minimizes the risk of using

You say slot 7 here, but the title of the patch says slot 4?

> +		 * the PAT bit as slot 3 is UC and is currently unused.
> +		 * Slot 4 should remain as reserved.
> +		 *
> +		 * PAT 0:WB, 1:WC, 2:UC-, 3:UC, 4-6:reserved, 7: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);
> +	}
>  
>  	/* Boot CPU check */
>  	if (!boot_pat_state)
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