On my system, error message:
[ 4.322008] dmar: DRHD: handling fault status reg 2
[ 4.327484] dmar: DMAR:[DMA Read] Request device [21:00.0] fault addr
fff66000
[ 4.327484] DMAR:[fault reason 01] Present bit in root entry is clear
fault happens on device : 21:00.0
To describe this problem clearly, we mark two time points:
A: Just before dump kernel init the DMAR/IOMMU structures.
B: Just after DMAR/IOMMU finished initialization.
C: Just before 21:00.0
When the first kernel crashed, then the dump kernel will boot.
Then there comes a DMA request on 21:00.0 when durting A to C. And this
will cause this error.
If I clear the root entry table when iommu structure is allocted, this
error will not happen during A to B, but still can be seen during B and
C. I guess the cause is the un-expected DMA request.
I will apply Bill Sumner's patch to see whether it will cause the errors
disappear.
-- Zhenhua
On 08/19/2014 07:02 PM, Joerg Roedel wrote:
On Mon, Aug 18, 2014 at 11:27:01PM +0000, Li, Zhen-Hua wrote:
: [fault reason 01] Present bit in root entry is clear
It appears when iommu initializing in the kdump kernel.
Hmm, do you have an explanation how this can happen? From how I read the
code, the kdump kernel disables translation on the IOMMU, then sets a
new root entry, and then re-enabled translation. To me it looks like
there is no point in time where translation is enabled and the
root-entry is clear (present-bit==0).
But obviously I am missing something if you see the message above.
Btw, have you looked into this patch-set posted earlier this year:
https://lkml.org/lkml/2014/4/24/836
It approaches the same problem-space, but also cares about in-flight
DMA.
Joerg
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