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 14:45:15 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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

On Fri, 2014-09-12 at 15:33 -0400, Konrad Rzeszutek Wilk wrote:
> > -	/* 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.

OK, I will try to preserve the picture.

> > -	 * 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?

Oops! I will fix the title.

Thanks for reviewing!
-Toshi


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