[PATCH] amd iommu: force flush of iommu prior during shutdown

Neil Horman nhorman at tuxdriver.com
Wed Mar 31 16:27:45 EDT 2010

On Wed, Mar 31, 2010 at 12:51:25PM -0700, Eric W. Biederman wrote:
> Neil Horman <nhorman at tuxdriver.com> writes:
> > On Wed, Mar 31, 2010 at 11:57:46AM -0700, Eric W. Biederman wrote:
> >> Neil Horman <nhorman at tuxdriver.com> writes:
> >> 
> >> > On Wed, Mar 31, 2010 at 11:54:30AM -0400, Vivek Goyal wrote:
> >> 
> >> >> So this call amd_iommu_flush_all_devices() will be able to tell devices
> >> >> that don't do any more DMAs and hence it is safe to reprogram iommu
> >> >> mapping entries.
> >> >> 
> >> > It blocks the cpu until any pending DMA operations are complete.  Hmm, as I
> >> > think about it, there is still a small possibility that a device like a NIC
> >> > which has several buffers pre-dma-mapped could start a new dma before we
> >> > completely disabled the iommu, althought thats small.  I never saw that in my
> >> > testing, but hitting that would be fairly difficult I think, since its literally
> >> > just a few hundred cycles between the flush and the actual hardware disable
> >> > operation.
> >> >
> >> > According to this though:
> >> > http://support.amd.com/us/Processor_TechDocs/34434-IOMMU-Rev_1.26_2-11-09.pdf
> >> > That window could be closed fairly easily, but simply disabling read and write
> >> > permissions for each device table entry prior to calling flush.  If we do that,
> >> > then flush the device table, any subsequently started dma operation would just
> >> > get noted in the error log, which we could ignore, since we're abot to boot to
> >> > the kdump kernel anyway.
> >> >
> >> > Would you like me to respin w/ that modification?
> >> 
> >> Disabling permissions on all devices sounds good for the new virtualization
> >> capable iommus.  I think older iommus will still be challenged.  I think
> >> on x86 we have simply been able to avoid using those older iommus.
> >> 
> >> I like the direction you are going but please let's put this in a
> >> paranoid iommu enable routine.
> >> 
> > You mean like initialize the device table so that all devices are default
> > disabled on boot, and then selectively enable them (perhaps during a
> > device_attach)?  I can give that a spin.
> That sounds good.

So I'm officially rescinding this patch.  It apparently just covered up the
problem, rather than solved it outright.  This is going to take some more
thought on my part.  I read the code a bit closer, and the amd iommu on boot up
currently marks all its entries as valid and having a valid translation (because
if they're marked as invalid they're passed through untranslated which strikes
me as dangerous, since it means a dma address treated as a bus address could
lead to memory corruption.  The saving grace is that they are marked as
non-readable and non-writeable, so any device doing a dma after the reinit
should get logged (which it does), and then target aborted (which should
effectively squash the translation)

I'm starting to wonder if:

1) some dmas are so long lived they start aliasing new dmas that get mapped in
the kdump kernel leading to various erroneous behavior


2) a slew of target aborts to some hardware results in them being in an
inconsistent state

I'm going to try marking the dev table on shutdown such that all devices have no
read/write permissions to see if that changes the situation.  It should I think
give me a pointer as to weather (1) or (2) is the more likely problem.

Lots more thinking to do....

> Eric
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