linux-kernel - Re: pci-gart_64.c: could we get better explanation?

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Message-Id: <1211831499.3286.2.camel@pasglop>
Date:	Tue, 27 May 2008 05:51:39 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Pavel Machek <pavel@...e.cz>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Dave Jones <davej@...emonkey.org.uk>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: pci-gart_64.c: could we get better explanation?


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.


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