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

[Daniel P . Berrangé]

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
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