linux-kernel - Re: [PATCH 3/4] Revert "x86: disable IOMMUs on kernel crash"

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Date:	Sun, 4 Apr 2010 10:44:36 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Joerg Roedel <joerg.roedel@....com>, nhorman@...hat.com,
	nhorman@...driver.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, hbabu@...ibm.com,
	iommu@...ts.linux-foundation.org, vgoyal@...hat.com
Subject: Re: [PATCH 3/4] Revert "x86: disable IOMMUs on kernel crash"

On Sat, Apr 03, 2010 at 10:44:22AM -0700, Eric W. Biederman wrote:
> Joerg Roedel <joro@...tes.org> writes:

> > Hmm, I think for this we need to change the gart code too and disable
> > the gart before its initialization runs to not re-introduce issues fixed
> > in commit bc2cea6a34fdb30f118ec75db39a46a191870607, no?
> 
> That is a different code path with a different set of assumptions and
> restrictions.  On a normal kexec of course we want to do an orderly shutdown.

Thats another problem with this patch. It introduces a difference
between the panic-shutdown kexec and the ordinary kexec.

> For the gart with a little luck we can just ignore it on kexec on
> panic.

The commit I mentioned above already proves this assumption wrong.

> Unlike a virtualization capable iommu it doesn't prevent access
> to devices, when it is enabled.   Worst case is that we have to start
> including iommu=off for gart systems.

No no no. This is a maintenance nightmare for almost everybody. Where do
you want to Document this special cases that 'if kernel uses gart then
and only then boot the kexec kernel with iommu=off'.
Always passing iommu=off to the kexec kernel doesn't work too for
obvious reasons.

> The best case is that we can figure out how to have the gart code
> reinitialize itself sanely, starting from some arbitrary point.

Yes, that is missing in this patch. But to keep changes small and don't
bother with the gart code at all I suggest to remove the shutdown
routine from the amd-iommu code only and not the whole shutdown call in
the machine_crash_shutdown path.

	Joerg

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