On Fri, Dec 13, 2024 at 08:46:42AM -0400, Jason Gunthorpe wrote: > 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.
Ok, that naming is destined to cause confusion for many, given the commonly understood use of 'nesting' in the context of VMs... > > > 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. ... describing it as a HW accelerated iommu makes it significantly clearer to me what this proposal is about. Perhaps the device is better named as "arm-smmuv3-accel" ? With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
