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

[Joerg Roedel <joro@8bytes.org>]

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