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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.12.14 at 02:07, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Tuesday, December 09, 2014 6:50 PM
>> 
>> >>> 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.
>> 
> 
> No. What we require is the raw MFNs. One IOMMU device entry can't
> point to multiple VM's page tables, so that's why XenGT needs to use
> software shadow GPU page table to implement the sharing. Note it's
> not for dom0 to access the MFN. It's for dom0 to setup the correct
> shadow GPU page table, so a VM can access the graphics memory
> in a controlled way.

So what's the translation flow here: driver -> GPU -> IOMMU ->
hardware or driver -> IOMMU -> GPU -> hardware? Or do things get
set up for the GPU to bypass the IOMMU altogether?

Jan


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