Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable nested SMMUv3

[Jason Gunthorpe]

On Fri, Dec 13, 2024 at 12:00:43PM +0000, Daniel P. Berrangé wrote:
> On Fri, Nov 08, 2024 at 12:52:37PM +0000, Shameer Kolothum via wrote:
> > Hi,
> > 
> > This series adds initial support for a user-creatable "arm-smmuv3-nested"
> > device to Qemu. At present the Qemu ARM SMMUv3 emulation is per machine
> > and cannot support multiple SMMUv3s.
> > 
> > In order to support vfio-pci dev assignment with vSMMUv3, the physical
> > SMMUv3 has to be configured in nested mode. Having a pluggable
> > "arm-smmuv3-nested" device enables us to have multiple vSMMUv3 for Guests
> > running on a host with multiple physical SMMUv3s. A few benefits of doing
> > this are,
> 
> I'm not very familiar with arm, but from this description I'm not
> really seeing how "nesting" is involved here. You're only talking
> about the host and 1 L1 guest, no L2 guest.

nesting is the term the iommu side is using to refer to the 2
dimensional paging, ie a guest page table on top of a hypervisor page
table.

Nothing to do with vm nesting.
 
> Also what is the relation between the physical SMMUv3 and the guest
> SMMUv3 that's referenced ? Is this in fact some form of host device
> passthrough rather than nesting ?

It is an acceeleration feature, the iommu HW does more work instead of
the software emulating things. Similar to how the 2d paging option in
KVM is an acceleration feature.

All of the iommu series on vfio are creating paravirtualized iommu
models inside the VM. They access various levels of HW acceleration to
speed up the paravirtualization.

Jason