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

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Date:	Wed, 31 Mar 2010 11:57:46 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Neil Horman <nhorman@...driver.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, iommu@...ts.linux-foundation.org,
	joerg.roedel@....com, hbabu@...ibm.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] amd iommu: force flush of iommu prior during shutdown

Neil Horman <nhorman@...driver.com> writes:

> On Wed, Mar 31, 2010 at 11:54:30AM -0400, Vivek Goyal wrote:

>> So this call amd_iommu_flush_all_devices() will be able to tell devices
>> that don't do any more DMAs and hence it is safe to reprogram iommu
>> mapping entries.
>> 
> It blocks the cpu until any pending DMA operations are complete.  Hmm, as I
> think about it, there is still a small possibility that a device like a NIC
> which has several buffers pre-dma-mapped could start a new dma before we
> completely disabled the iommu, althought thats small.  I never saw that in my
> testing, but hitting that would be fairly difficult I think, since its literally
> just a few hundred cycles between the flush and the actual hardware disable
> operation.
>
> According to this though:
> http://support.amd.com/us/Processor_TechDocs/34434-IOMMU-Rev_1.26_2-11-09.pdf
> That window could be closed fairly easily, but simply disabling read and write
> permissions for each device table entry prior to calling flush.  If we do that,
> then flush the device table, any subsequently started dma operation would just
> get noted in the error log, which we could ignore, since we're abot to boot to
> the kdump kernel anyway.
>
> Would you like me to respin w/ that modification?

Disabling permissions on all devices sounds good for the new virtualization
capable iommus.  I think older iommus will still be challenged.  I think
on x86 we have simply been able to avoid using those older iommus.

I like the direction you are going but please let's put this in a
paranoid iommu enable routine.

Eric
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