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

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Message-Id: <1256192928.2990.10.camel@macbook.infradead.org>
Date:	Thu, 22 Oct 2009 15:28:48 +0900
From:	David Woodhouse <dwmw2@...radead.org>
To:	Alex Williamson <alex.williamson@...com>
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 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.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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