[PATCH v1 02/14] iommufd: Add nesting related data structures for ARM SMMUv3

[Tian, Kevin]

> From: Robin Murphy <robin.murphy at arm.com>
> Sent: Friday, March 10, 2023 11:57 PM
> 
> >
> > If iommufd could provide a general cross-driver API to set exactly
> > that scenario up then VMM code could also be general. That seems
> > prettty interesting.
> 
> Indeed, I've always assumed the niche for virtio would be that kind of
> in-between use-case using nesting to accelerate simple translation,
> where we plug a guest-owned pagetable into a host-owned context. That
> way the guest retains the simple virtio interface and only needs to
> understand a pagetable format (or as you say, simply share a CPU
> pagetable) without having to care about the nitty-gritty of all the
> IOMMU-specific moving parts around it. For guests that want to get into
> more advanced stuff like managing their own PASID tables, pushing them
> towards "native" nesting probably does make more sense.
> 

Interesting thing is that we cannot expose both virtio-iommu and
emulated vIOMMU to one guest to choose. then if the guest has
been using virtio-iommu for whatever reason naturally it may
want more advanced features on it too.