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Re: [Xen-devel] iommu=dom0-passthrough behavior

Jan Beulich wrote on 2012-11-05:
> All,
> so far it was my understanding that this option is intended to get
> the DMA behavior that Dom0 observes as close as possible to how
> it would be without IOMMU.

Correct. There is a bit in context entry which controlling the DMA request(from 
this device) to walk or not walk the iommu page table.
As we known, walking page table introduced extra cost, so we use this parameter 
to make sure the device which owned by dom0 not to walking iommu page table 
when DMA request is arrived.
> However, we're now dealing with a customer report where a
> single function device is observed to initiate DMA operations
> appearing to originate from function 1, which makes obvious that
> the option above is not making things as transparent as I would
> have expected them to be: Without IOMMU, such requests get
> processed fine, while with IOMMU (due to there not being a
> context entry for the bogus device) the device fails to initialize
> (causing DMA faults, the presence of which I had to convince
> myself of separately, as for whatever reason at least the VT-d
> code doesn't issue any log message in that case).

Sorry, I cannot understand your problem. Is there any bug in current VT-d code?

> So I'm now seeking for alternative workaround suggestions that
> we could pass to that customer (less intrusive than "iommu=off").

Best regards,

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