linux-kernel - Re: [PATCH] [4/7] Don't use large pages to map the first 2/4MB of memory

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Message-ID: <alpine.LFD.1.00.0803211843120.3781@apollo.tec.linutronix.de>
Date:	Fri, 21 Mar 2008 18:45:54 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	andreas.herrmann3@....com, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [4/7] Don't use large pages to map the first 2/4MB of
 memory

On Wed, 12 Mar 2008, Andi Kleen wrote:
> Intel recommends to not use large pages for the first 1MB 
> of the physical memory because there are fixed size MTRRs there
> which cause splitups in the TLBs.
> 
> On AMD doing so is also a good idea.
> 
> The implementation is a little different between 32bit and 64bit.
> On 32bit I just taught the initial page table set up about this
> because it was very simple to do. This also has the advantage
> that the risk of a prefetch ever seeing the page even
> if it only exists for a short time is minimized.
> 
> On 64bit that is not quite possible, so use set_memory_4k() a little
> later (in check_bugs) instead.


> +	/*
> +	 * Make sure the first 2MB area is not mapped by huge pages
> +	 * There are typically fixed size MTRRs in there and overlapping
> +	 * MTRRs into large pages causes slow downs.
> +	 *
> +	 * Right now we don't do that with gbpages because there seems
> +	 * very little benefit for that case.

And why exactly ? Does slowdown not matter with gbpages ? 

Also we split the first GB mapping anyway due to the various regions
(NX, RO, UC) in there.

Thanks,

	tglx
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