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

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Date:	Wed, 03 Apr 2013 09:24:39 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Takao Indoh <indou.takao@...fujitsu.com>
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

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?

>  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*. :)

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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