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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.12.14 at 11:37, <yu.c.zhang@xxxxxxxxxxxxxxx> 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.

Hmm, that suggests you indeed need raw MFNs, which in turn seems
problematic wrt PVH Dom0 (or you'd need a GFN->GMFN translation
layer). But while you don't use the IOMMU yourself, I suppose the GPU
accesses still don't bypass the IOMMU? In which case all you'd need
returned is a frame number that guarantees that after IOMMU
translation it refers to the correct MFN, i.e. still allowing for your Dom0
driver to simply set aside a part of its PFN space, asking Xen to
(IOMMU-)map the necessary guest frames into there.

Jan


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