linux-kernel - Re: [PATCH] intel-iommu: Fix alloc_coherent for pass-through devices

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Date:	Thu, 22 Oct 2009 06:24:01 -0600
From:	Alex Williamson <alex.williamson@...com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	iommu@...ts.linux-foundation.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Miller, Mike (OS Dev)" <mike.miller@...com>
Subject: Re: [PATCH] intel-iommu: Fix alloc_coherent for pass-through
 devices

On Thu, 2009-10-22 at 15:28 +0900, David Woodhouse wrote:
> On Wed, 2009-10-21 at 21:41 -0600, Alex Williamson wrote:
> > After 19943b0e (intel-iommu: Unify hardware and software passthrough
> > support) hardware pass-through mode devices make use of intel_dma_ops
> > rather than swiotlb_dma_ops.  The problem is that intel_alloc_coherent
> > ignores the device coherent_dma_mask when allocating the page since it
> > expects to remap the page and provide the device with an iova within the
> > coherent mask.  This breaks when we use pass-through.
> > 
> > The patch below crudely works around the problem, but I hope we can come
> > up with something better without reintroducing the dependency on
> > swiotlb.  The device hitting this problem is an HP smart array
> > controller on a Proliant G6 system.  It uses a default 32bit coherent
> > DMA mask, and stalls, presumably waiting on control data to change in
> > the wrong address space, when it gets a coherent buffer above 4G.  This
> > device also doesn't exactly play nice when using VT-d in anything other
> > than pass-through mode, so switching it into mapped mode is not really
> > an option.
> 
> I don't understand. If your device can't cope with 64-bit addresses,
> then surely iommu_no_mapping() should return _false_ for it. And we'll
> actually use the IOMMU even though we're generally in passthrough mode.

The coherent_dma_mask is independent of the dma_mask and can be set
separately by the device.  The default for any device that doesn't
specify one is 32bits.  iommu_should_identity_map() only checks the
dma_mask, not the coherent_dma_mask.  We never see "32bit %s uses
non-identity mapping" for this device, and I suspect if we did it also
wouldn't work because of the mentioned unfriendliness in mapped mode.
Since coherent mappings are generally a slow path, I think we should be
making more effort to get a usable buffer before dumping the device out
of passthrough mode.  BTW, we skip RMRR setup when doing hardware
pass-through, but I can't find where they get reloaded if we then end up
removing the device from the si_domain.  Is this another issue?  Thanks,

Alex


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