Re: [PATCH v9 0/10] iommu/vt-d: Fix intel vt-d faults in kdump kernel

From: Don Dutile
Date: Sun May 10 2015 - 21:31:59 EST

On 05/07/2015 10:00 AM, Dave Young wrote:
On 04/07/15 at 10:12am, Don Dutile wrote:
On 04/06/2015 11:46 PM, Dave Young wrote:
On 04/05/15 at 09:54am, Baoquan He wrote:
On 04/03/15 at 05:21pm, Dave Young wrote:
On 04/03/15 at 05:01pm, Li, ZhenHua wrote:
Hi Dave,

There may be some possibilities that the old iommu data is corrupted by
some other modules. Currently we do not have a better solution for the
dmar faults.

But I think when this happens, we need to fix the module that corrupted
the old iommu data. I once met a similar problem in normal kernel, the
queue used by the qi_* functions was written again by another module.
The fix was in that module, not in iommu module.

It is too late, there will be no chance to save vmcore then.

Also if it is possible to continue corrupt other area of oldmem because
of using old iommu tables then it will cause more problems.

So I think the tables at least need some verifycation before being used.

Yes, it's a good thinking anout this and verification is also an
interesting idea. kexec/kdump do a sha256 calculation on loaded kernel
and then verify this again when panic happens in purgatory. This checks
whether any code stomps into region reserved for kexec/kernel and corrupt
the loaded kernel.

If this is decided to do it should be an enhancement to current
patchset but not a approach change. Since this patchset is going very
close to point as maintainers expected maybe this can be merged firstly,
then think about enhancement. After all without this patchset vt-d often
raised error message, hung.

It does not convince me, we should do it right at the beginning instead of
introduce something wrong.

I wonder why the old dma can not be remap to a specific page in kdump kernel
so that it will not corrupt more memory. But I may missed something, I will
looking for old threads and catch up.


The (only) issue is not corruption, but once the iommu is re-configured, the old,
not-stopped-yet, dma engines will use iova's that will generate dmar faults, which
will be enabled when the iommu is re-configured (even to a single/simple paging scheme)
in the kexec kernel.

Don, so if iommu is not reconfigured then these faults will not happen?

Baoquan and me has a confusion below today about iommu=off/intel_iommu=off:



disable active iommu translations;

if (no_iommu || dmar_disabled)
goto out_free_dmar;


Any reason not move no_iommu check to the begining of intel_iommu_init function?

Looks like you could.

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