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Re: [Xen-devel] [PATCH v7 2/4] xen/arm: grant: Add another entry to map MFN 1:1 in dom0 p2m

On 15/05/14 15:16, Julien Grall wrote:
Grant mapping can be used for DMA request. The dev_bus_addr returned by the
hypercall is the MFN (not the IPA). Currently Linux is using this address (via
swiotlb) to program the DMA.
When the device is protected by IOMMU the request will fail. We have to
add 1:1 mapping in the domain p2m to allow DMA request working.

This is valid because DOM0 has its memory mapped 1:1 and therefore we know
that RAM and devices cannot clash.

I though a bit more about this patch. It doesn't handle the case where 2 grant use the same MFN, which is valid.

In this case, we should remove the 1:1 mapping only when the last grant is removed.

I'm not sure how to handle this case.


Julien Grall

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