RE: intel-iommu.c bug

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Fri, 2010-12-17 at 10:11 -0800, Raj, Ashok wrote:
> Hi Cliff
> 
> Sending this to Suresh who is handling linux iommu.

No. It is David Woodhouse (added).

> 
> 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...
> >