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SubjectRE: intel-iommu.c bug
Hi Cliff

Sending this to Suresh who is handling linux iommu.

Ashok


>-----Original Message-----
>From: Cliff Wickman [mailto:cpw@sgi.com]
>Sent: Friday, December 17, 2010 7:03 AM
>To: linux-kernel@vger.kernel.org
>Cc: Raj, Ashok; Li, Shaohua; Keshavamurthy, Anil S; Yu, Fenghua
>Subject: intel-iommu.c bug
>
>
>
>This bug was reported by Mike Habeck <habeck@sgi.com>.
>The test system was an SGI Altix UV. These are Mike's words:
>
> It appears there is a bug in the iommu code that when 'forcedac' isn't used
> the nvidia driver is handed back a 44bit dma address even though it's
> dma_mask is set to 40bits.
>
> I added some debug to the intel_iommu code and I see:
> intel_map_sg(): dma_addr_t=0xf81fffff000, pdev->dma_mask=0xffffffffff
>
> Note the dma_addr_t being handed back is 44bits even though the mask is 40bits.
> This results in the nvidia card generating a bad dma (i.e. the nvidia hw is
> only capable of generating a 40bit dma address so the upper 4 bits are lost
> and that results in the iommu hw detecting a bad dma access):
>
> DRHD: handling fault status reg 2
> DMAR:[DMA Read] Request device [36:00.0] fault addr 81fffff000
> DMAR:[fault reason 06] PTE Read access is not set
>
> If I boot with 'forcedac' then the dma mask is honored and the dma_addr_t
> handed back is 40bits:
>
> intel_map_sg(): dma_addr_t=0xfffffff000, pdev->dma_mask=0xffffffffff
>
> Without forcedac you'd expect these early maps being handed back to be 32bits.
> This is the first debug printf (so the first mapping the nvidia device has
> requested) so I'd expect it to be 0xfffff000... interesting that is what the
> lower 32bits are in this address being handed back... that 0xf81 upper bits
> appear to be garbage bits. This might be a hint to help find the bug...
>


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