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

[Julien Grall <julien.grall@xxxxxxxxxx>]

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.

Regards,

--
Julien Grall

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