Re: [PATCH] x86-64: disable the GART before allocate aperture

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

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 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).

Alan
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