linux-kernel - Re: why does the macro "ZERO

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Message-ID: <4668412D.3030506@zytor.com>
Date:	Thu, 07 Jun 2007 10:32:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
CC:	Satyam Sharma <satyam.sharma@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: why does the macro "ZERO_PAGE" take an argument?

Robert P. J. Day wrote:
>> The comment above empty_zero_page and zero_page_mask
>> declarations at arch/mips/mm/init.c:508 sheds light on this ...
> 
> well, it *sort of* does.  at line 64 of that file:
> 
> /*
>  * We have up to 8 empty zeroed pages so we can map one of the right colour
>  * when needed.  This is necessary only on R4000 / R4400 SC and MC versions
>  * where we have to avoid VCED / VECI exceptions for good performance at
>  * any price.  Since page is never written to after the initialization we
>  * don't have to care about aliases on other CPUs.
>  */
> 
> although it's not clear where in the source tree are the invocations
> that would actually make a difference to a MIPS system, which is why
> i've CC'ed ralf on this.  i'm sure he can clear this up. :-)

x86 could also benefit from coloured zeropages.  In fact, I thought it
already had them (K8 wants as many as 8.)

	-hpa
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