Re: [Xen-devel] One question about the hypercall to translate gfn to mfn.

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Malcolm Crossley
> Sent: Tuesday, December 09, 2014 6:52 PM
> 
> On 09/12/14 10:37, Yu, Zhang wrote:
> >
> >
> > On 12/9/2014 6:19 PM, Paul Durrant wrote:
> >> I think use of an raw mfn value currently works only because dom0 is
> >> using a 1:1 IOMMU mapping scheme. Is my understanding correct, or do
> >> you really need raw mfn values?
> > Thanks for your quick response, Paul.
> > Well, not exactly for this case. :)
> > In XenGT, our need to translate gfn to mfn is for GPU's page table,
> > which contains the translation between graphic address and the memory
> > address. This page table is maintained by GPU drivers, and our service
> > domain need to have a method to translate the guest physical addresses
> > written by the vGPU into host physical ones.
> > We do not use IOMMU in XenGT and therefore this translation may not
> > necessarily be a 1:1 mapping.
> 
> XenGT must use the IOMMU mappings that Xen has setup for the domain
> which owns the GPU. Currently Dom0 own's the GPU and so it's IOMMU
> mappings match the MFN's addresses. I suspect XenGT will not work if Xen
> is booted with iommu=dom0-strict.
> 

This is a good point. So yes in this case IOMMU is still active which contains
a 1:1 IOMMU mapping table, but it's a separate thing from the interface
discussed here, which is about setup a shadow GPU page table for other VM's
graphics memory accesses. 

Thanks
Kevin

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