Re: [Xen-devel] [PATCH v8 4/4] xen/arm: grant: Add another entry to map MFN 1:1 in dom0 p2m

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-05-20 at 08:46 +0100, Jan Beulich wrote:
> >>> On 19.05.14 at 18:24, <julien.grall@xxxxxxxxxx> 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.
> 
> Wasn't this 1:1 mapping meant as a temporary workaround? If so,
> does it really make sense to base further code on it?

IOMMU on ARM has not become so prevalent (yet?) that we can get rid of
it. I think we are mostly resigned to keeping it around for the time
being. (sadly)

Ian.


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