linux-kernel - Re: [PATCH] amd iommu: force flush of iommu prior during shutdown

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Message-ID: <20100401155643.GG24846@8bytes.org>
Date:	Thu, 1 Apr 2010 17:56:43 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	hbabu@...ibm.com, iommu@...ts.linux-foundation.org,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] amd iommu: force flush of iommu prior during shutdown

On Thu, Apr 01, 2010 at 10:47:36AM -0400, Neil Horman wrote:
> On Thu, Apr 01, 2010 at 04:29:02PM +0200, Joerg Roedel wrote:
> > I am back in office next tuesday and will look into this problem too.
> > 
> Thank you.

Just took a look and I think the problem is that the devices are
attached to domains before the IOMMU hardware is enabled. This happens
in the function prealloc_protection_domains(). The attach code issues
the dte-invalidate commands but they are not executed because the
hardware is off. I will verify this when I have access to hardware
again.
The possible fix will be to enable the hardware earlier in the
initialization path.

> > Right. The default for all devices is to forbid DMA.
> > 
> Thanks, glad to know I read that right, took me a bit to understand it :)

I should probably add a comment :-)

> > Thats indeed true. I have seen that with ixgbe cards for example. They
> > seem to be really confused after an target abort.
> > 
> Yeah, this part worries me, target aborts lead to various brain dead hardware
> pieces.  What are you thoughts on leaving the iommu on through a reboot to avoid
> this issue (possibly resetting any pci device that encounters a target abort, as
> noted in the error log on the iommu?

This would only prevent possible data corruption. When the IOMMU is off
the devices will not get a target abort but will only write to different
physical memory locations. The window where a target abort can happen
starts when the kdump kernel re-enables the IOMMU and ends when the new
driver for that device attaches. This is a small window but there is not
a lot we can do to avoid this small time window.

	Joerg

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