Re: [Xen-devel] [PATCH v5 09/15] vtd: add lookup_page method to iommu_ops

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

> From: Paul Durrant [mailto:Paul.Durrant@xxxxxxxxxx]
> Sent: Tuesday, August 7, 2018 4:36 PM
> 
> > > > > +static int intel_iommu_lookup_page(struct domain *d, bfn_t bfn,
> > > mfn_t
> > > > > *mfn,
> > > > > +                                   unsigned int *flags)
> > > >
> > > > Not looking at later patches yet... but in concept bfn address
> > > > space is per device instead of per domain.
> > >
> > > Not in this case. Xen has always maintained a single IOMMU address
> per
> > > virtual machine. That is what BFN refers to.
> > >
> > >   Paul
> > >
> > > > In default situation
> > > > (w/o pvIOMMU exposed), all devices assigned to dom0 share
> > > > the same address space (bfn=pfn) which is currently linked
> > > > from domain structure. Then with pvIOMMU exposed, dom0
> > > > starts to manage individual pfn address space (called IOVA
> > > > address space within dom0) per assigned device. In that case
> > > > lookup should accept a bdf number and then find the right
> > > > page table...
> > > >
> > >
> > > No. That is over-complicating things and would probably involve re-
> writing
> > > most of the IOMMU code in Xen AFAICT.
> > >
> >
> > it's not over-complicating. it's about correctness.
> >
> > think about the role of IOMMU. Now you expose it to dom0. Then
> > dom0 wants to use it to isolate devices. What is the matter having
> > an IOMMU which supports only one global address space cross all
> > devices, from dom0's p.o.v? If that is the case, why does dom0 ever
> > want to change bfn globally on all devices?
> >
> 
> Because even a PV dom0 would be beter off with a BFN:GFN map rather
> than a BFN:MFN map, since then the xen_swiotlb code and be avoided and
> lots of things that are contiguous in GFN space become contiguous in BFN
> space and hence become DMA-able without bouncing.
> 

so this pvIOMMU really loses most functionalities which a physical IOMMU
provides... maybe not call it pvIOMMU. sort of pvDMA engine.

Thanks
Kevin

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