linux-kernel - Re: [PATCH 1/1] iommu/vt-d : clear old root entry for dump kernel

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Date:	Wed, 20 Aug 2014 17:54:43 +0800
From:	"Li, ZhenHua" <zhen-hual@...com>
To:	Joerg Roedel <joro@...tes.org>
CC:	David Woodhouse <dwmw2@...radead.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linda Knippers <linda.knippers@...com>
Subject: Re: [PATCH 1/1] iommu/vt-d : clear old root entry for dump kernel


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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