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Subject: Re: pci-gart_64.c: could we get better explanation?
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Pavel Machek <pavel@suse.cz>
Cc: kernel list <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Dave Jones <davej@codemonkey.org.uk>, Andi Kleen <andi@firstfloor.org>
In-Reply-To: <20080526184703.GA17369@elf.ucw.cz>
References: <20080526184703.GA17369@elf.ucw.cz>
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Date: Tue, 27 May 2008 05:51:39 +1000
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On Mon, 2008-05-26 at 20:47 +0200, Pavel Machek wrote:
> Hi!
> 
> +++ b/arch/x86/kernel/pci-gart_64.c
> @@ -774,6 +778,7 @@ #endif
>  
>         /*
>          * Try to workaround a bug (thanks to BenH)
>          * Set unmapped entries to a scratch page instead of 0.
>          * Any prefetches that hit unmapped entries won't get an bus abort
> 
> Could someone explain? Which bug is being worked around? CPUs
> prefetching over NULL?

P2P bridge prefetching on DMA reads.

Ben.