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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Sat, 2014-05-17 at 19:12 +0100, Julien Grall wrote:
> 
> 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.

Urk, yes indeed.

This is all hypervisor side stuff I think, right?

I wonder if the grant table maptrack stuff is helpful here, i.e. could
it contain a reference count (if it doesn't already)? Some care and
attention would be needed to make it safe, but it might be doable.

Ian.


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