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

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Message-ID: <20100331191811.GD13406@hmsreliant.think-freely.org>
Date:	Wed, 31 Mar 2010 15:18:11 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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

On Wed, Mar 31, 2010 at 11:57:46AM -0700, Eric W. Biederman wrote:
> 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.
> 
You mean like initialize the device table so that all devices are default
disabled on boot, and then selectively enable them (perhaps during a
device_attach)?  I can give that a spin.
Neil

> Eric
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