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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.12.14 at 09:47, <kevin.tian@xxxxxxxxx> wrote:
> two translation paths in assigned case:
> 
> 1. [direct CPU access from VM], with partitioned PCI aperture
> resource, every VM can access a portion of PCI aperture directly.
> 
> - CPU page table/EPT: CPU virtual address->PCI aperture
> - PCI aperture - bar base = Graphics Memory Address (GMA)
> - GPU page table: GMA -> GPA (as programmed by guest)
> - IOMMU: GPA -> MPA
> 
> 2. [GPU access through GPU command operands], with GPU scheduling,
> every VM's command buffer will be fetched by GPU in a time-shared
> manner.
> 
> - GPU page table: GMA->GPA
> - IOMMU: GPA->MPA
> 
> In our case, IOMMU is setup with 1:1 identity table for dom0. So 
> when GPU may access GPAs from different VMs, we can't count on
> IOMMU which can only serve one mapping for one device (unless 
> we have SR-IOV). 
> 
> That's why we need shadow GPU page table in dom0, and need a
> p2m query call to translate from GPA -> MPA:
> 
> - shadow GPU page table: GMA->MPA
> - IOMMU: MPA->MPA (for dom0)

I still can't see why the Dom0 translation has to remain 1:1, i.e.
why Xen couldn't return some "arbitrary" GPA for the query in
question here, setting up a suitable GPA->MPA translation. (I put
arbitrary in quotes because this of course must not conflict with
GPAs already or possibly in use by Dom0.) And I can only stress
again that you shouldn't leave out PVH (where the IOMMU already
isn't set up with all 1:1 mappings) from these considerations.

Jan


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