linux-kernel - Re: [PATCH 0/8] iommu/vt-d: Fix crash dump failure caused by legacy DMA/IO

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Date:	Fri, 11 Jul 2014 10:27:45 -0600
From:	Jerry Hoemann <jerry.hoemann@...com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	David Woodhouse <dwmw2@...radead.org>, indou.takao@...fujitsu.com,
	bhe@...hat.com, linux-pci@...r.kernel.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, ishii.hironobu@...fujitsu.com,
	bhelgaas@...gle.com, zhenhua@...com
Subject: Re: [PATCH 0/8] iommu/vt-d: Fix crash dump failure caused by legacy
 DMA/IO

On Wed, Jul 02, 2014 at 03:32:59PM +0200, Joerg Roedel wrote:
> Hi David,
> 
> On Wed, Apr 30, 2014 at 11:49:33AM +0100, David Woodhouse wrote:
> > There could be all kinds of existing mappings in the DMA page tables,
> > and I'm not sure it's safe to preserve them. What prevents the crashdump
> > kernel from trying to use any of the physical pages which are
> > accessible, and which could thus be corrupted by stray DMA?
> > 
> > In fact, the old kernel could even have set up 1:1 passthrough mappings
> > for some devices, which would then be able to DMA *anywhere*. Surely we
> > need to prevent that?
> 
> Ideally we would prevent that, yes. But the problem is that a failed DMA
> transaction might put the device into an unrecoverable state. Usually
> any in-flight DMA transactions should only target buffers set up by the
> previous kernel and not corrupt any data.
> 
> > After the last round of this patchset, we discussed a potential
> > improvement where you point every virtual bus address at the *same*
> > physical scratch page.
> 
> That is a solution to prevent the in-flight DMA failures. But what
> happens when there is some in-flight DMA to a disk to write some inodes
> or a new superblock. Then this scratch address-space may cause
> filesystem corruption at worst.
> 
> So with this in mind I would prefer initially taking over the
> page-tables from the old kernel before the device drivers re-initialize
> the devices.
> 
> 
> 	Joerg

David, Joerg,

What do you think here?  Do you want me to update the patch set for 3.17?

Jerry

-- 

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Jerry Hoemann            Software Engineer              Hewlett-Packard

3404 E Harmony Rd. MS 57                        phone:  (970) 898-1022
Ft. Collins, CO 80528                           FAX:    (970) 898-XXXX
                                                email:  jerry.hoemann@...com
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