Re: [RFC PATCH v2 00/58] KVM: Arm SMMUv3 driver for pKVM

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jan 16, 2025 at 06:39:31AM +0000, Tian, Kevin wrote:
> > From: Mostafa Saleh <smostafa@google.com>
> > Sent: Wednesday, January 8, 2025 8:10 PM
> > 
> > On Thu, Jan 02, 2025 at 04:16:14PM -0400, Jason Gunthorpe wrote:
> > > On Fri, Dec 13, 2024 at 07:39:04PM +0000, Mostafa Saleh wrote:
> > > > Yeah, SVA is tricky, I guess for that we would have to use nesting,
> > > > but tbh, I don’t think it’s a deal breaker for now.
> > >
> > > Again, it depends what your actual use case for translation is inside
> > > the host/guest environments. It would be good to clearly spell this out..
> > > There are few drivers that directly manpulate the iommu_domains of a
> > > device. a few gpus, ath1x wireless, some tegra stuff, "venus". Which
> > > of those are you targetting?
> > >
> > 
> > Not sure I understand this point about manipulating domains.
> > AFAIK, SVA is not that common, including mobile spaces but I can be wrong,
> > that’s why it’s not a priority here.
> 
> Nested translation is required beyond SVA. A scenario which requires
> a vIOMMU and multiple device domains within the guest would like to
> embrace nesting. Especially for ARM vSMMU nesting is a must.

Right, if you need an iommu domain in the guest there are only three
mainstream ways to get this in Linux:
 1) Use the DMA API and have the iommu group be translating. This is
    optional in that the DMA API usually supports identity as an option.
 2) A driver directly calls iommu_paging_domain_alloc() and manually
    attaches it to some device, and does not use the DMA API. My list
    above of ath1x/etc are examples doing this
 3) Use VFIO

My remark to Mostafa is to be specific, which of the above do you want
to do in your mobile guest (and what driver exactly if #2) and why.

This will help inform what the performance profile looks like and
guide if nesting/para virt is appropriate.

> But I'm not sure that I got Jason's point about " there is no way to get
> SVA support with para-virtualization." virtio-iommu is a para-virtualized
> model and SVA support is in its plan. The main requirement is to pass
> the base pointer of the guest CPU page table to backend and PRI faults/
> responses back forth.

That's nesting, you have a full page table under the control of the
guest, and the guest needs to have a level of HW-specific
knowledge. It is just an alternative to using the native nesting
vIOMMU.

What I mean by "para-virtualization" is the guest does map/unmap calls
to the hypervisor and has no page tbale.

Jason