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From: Andi Kleen <ak@suse.de>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Subject: Re: [PATCH] x86-64: disable the GART before allocate aperture
Date: Fri, 22 Jun 2007 23:28:20 +0200
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Cc: Muli Ben-Yehuda <muli@il.ibm.com>, Yinghai Lu <Yinghai.Lu@sun.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       "Eric W. Biederman" <ebiederm@xmission.com>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Friday 22 June 2007 22:33, Alan Cox wrote:
> On Fri, 22 Jun 2007 12:31:24 -0700
>
> Muli Ben-Yehuda <muli@il.ibm.com> wrote:
> > On Fri, Jun 22, 2007 at 12:19:15PM -0700, Yinghai Lu wrote:
> > > [PATCH] x86-64: disable the GART before allocate aperture
> > >
> > > For K8 system: 4G RAM with memory hole remapping enabled, or more
> > > than 4G RAM installed. when mem is allocated for GART, it will do
> > > the memset for clear.  and for kexec case, the first kernel already
> > > enable that, the memset in second kernel will cause the system
> > > restart. So disable that at first before we try to allocate mem for
> > > it.
> >
> > Why does the memset in the second kernel cause a system restart?
>
> You've got mapped live gart pages from the previous kernel. Even if you
> disable the gart before a memset 

It's probably too late then. It could also interfere with other operations.
If anything the GART should be disabled during kexec shutdown. Perhaps we just 
need a suitable suspend function that does that. Eric, any preferences? 

> you may well have the video card using 
> gart translations and possibly live IOMMU mappings for devices using it
> via bus mastering - and those will cause you MCE exceptions with a
> corrupt cpu context flag (ie not nicely recoverable).

We disable those machine checks on K8 because they're not fully reliable.

-Andi