linux-kernel - Re: [PATCH] intel-iommu: Obey coherent_dma_mask for alloc

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Date:	Mon, 09 Nov 2009 23:02:27 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	alex.williamson@...com, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] intel-iommu: Obey coherent_dma_mask for alloc_coherent
 on	passthrough

On Fri, 2009-11-06 at 11:41 +0900, FUJITA Tomonori wrote:
> This is fine for 2.6.32 but we'll cleanly fix this by using
> swiotlb_dma_ops later, right?

Well, the idea was that with 'iommu=pt' we'd have passthrough mode for
_decent_ devices, but the crappy devices without 64-bit DMA would just
have the IOMMU enabled instead.

We can see this as a simple classification bug -- we should be checking
pdev->coherent_dma_mask as well as pdev->dma_mask:

--- a/drivers/pci/intel-iommu.c
+++ b/drivers/pci/intel-iommu.c
@@ -2196,7 +2196,8 @@ static int iommu_should_identity_map(struct pci_dev *pdev, int startup)
 	 * take them out of the 1:1 domain later.
 	 */
 	if (!startup)
-		return pdev->dma_mask > DMA_BIT_MASK(32);
+		return (pdev->dma_mask & pdev->coherent_dma_mask) < 
+			dma_get_required_mask();

 	return 1;
 }

That fixes the case of a 32-bit coherent_dma_mask as it was intended to
be fixed.

Unfortunately, Alex's hardware is more broken than that. It also likes
to do stray reads from unmapped addresses -- addresses which used to be
mapped at some time in the past. So he really _does_ want the IOMMU
disabled, or in passthrough mode.

But I think that's a special case and needs to be handled with a quirk,
while the above patch actually addresses the problem we claimed we were
trying to address.

Handling Alex's broken hardware probably wants to be done with
'iommu=off' for now, and then when Chris's swiotlb fallback patches are
done we can perhaps do something more cunning.

I'm slightly reluctant to put the half-arsed 'try to allocate in the
right region for broken devices but without full swiotlb support' option
into 2.6.32.

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

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