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

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

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, December 10, 2014 5:17 PM
> 
> >>> 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.
> 

It's interesting that you think IOMMU can be used in such situation.

what do you mean by "arbitrary" GPA here? and It's not just about 
conflicting with Dom0's GPA, it's about confliction in all VM's GPAs 
when you hosting them through one IOMMU page table, and there's 
no way to prevent this definitely since GPAs are picked by VMs 
themselves.

I don't think we can support PVH here if IOMMU is not 1:1 mapping.

Thanks
Kevin

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel