linux-kernel - Re: [PATCH] intel-iommu: Synchronize gcmd value with global command register

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Date:	Thu, 04 Apr 2013 14:48:25 +0900
From:	Takao Indoh <indou.takao@...fujitsu.com>
To:	dwmw2@...radead.org
CC:	joro@...tes.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org, kexec@...ts.infradead.org
Subject: Re: [PATCH] intel-iommu: Synchronize gcmd value with global command
 register

(2013/04/03 17:24), David Woodhouse wrote:
> On Wed, 2013-04-03 at 16:11 +0900, Takao Indoh wrote:
>> (2013/04/02 23:05), Joerg Roedel wrote:
>>> On Mon, Apr 01, 2013 at 02:45:18PM +0900, Takao Indoh wrote:
>>>> <Current flow on kdump boot>
>>>> enable_IR
>>>>     intel_enable_irq_remapping
>>>>       iommu_disable_irq_remapping  <== IRES/QIES/TES disabled here
>>>>       dmar_disable_qi              <== do nothing
>>>>       dmar_enable_qi               <== QIES enabled
>>>>       intel_setup_irq_remapping    <== IRES enabled
>>>
>>> But what we want to do here in the kdumo case is to disable translation
>>> too, right? Because the former kernel might have translation and
>>> irq-remapping enabled and the kdump kernel might be compiled without
>>> support for dma-remapping. So if we don't disable translation here too
>>> the kdump kernel is unable to do DMA.
>>
>> Yeah, you are right. I forgot such a case.
> 
> If you disable translation and there's some device still doing DMA, it's
> going to scribble over random areas of memory. You really want to have
> translation enabled and all the page tables *cleared*, during kexec. I
> think it's fair to insist that the secondary kernel should use the IOMMU
> if the first one did.
> 
>> To be honest, I also expected the side effect of this patch. As I wrote
>> in the previous mail, I'm working on kdump problem with iommu, that is,
>> ongoing DMA causes DMAR fault in 2nd kernel and sometimes kdump fails
>> due to this fault.
> 
> Here you've lost me. The DMAR fault is caught and reported, and how does
> this lead to a kdump failure? Are you using dodgy hardware that just
> keeps *trying* after an abort, and floods the system with a storm of
> DMAR faults? We've occasionally spoken about working around such a
> problem by setting a bit to make subsequent faults *silent*. Would that
> work?

There are several cases.
- DMAR fault messages floods and second kernel does not boot. Recently I
  saw similar report. https://lkml.org/lkml/2013/3/8/120
- igb driver detectes error on linkup and kdump via network fails.
- On a certain platform, though kdump itself works, PCIe error like
  Unexpected Completion is detected and it gets hardware degraded.

Thanks,
Takao Indoh


> 
>>   What we have to do is stopping DMA transaction
>> before DMA-remapping is disabled in 2nd kernel.
> 
> The IOMMU is there to stop DMA transactions. That is its *job*. :)
> 


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