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

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Message-ID: <20140819110239.GA16329@8bytes.org>
Date:	Tue, 19 Aug 2014 13:02:39 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"Li, Zhen-Hua" <zhen-hual@...com>
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>
Subject: Re: [PATCH 1/1] iommu/vt-d : clear old root entry for dump kernel

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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