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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Daniel P. Berrangé <berrange@redhat.com>
> Sent: Friday, December 13, 2024 1:20 PM
> To: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi@huawei.com>; qemu-arm@nongnu.org;
> qemu-devel@nongnu.org; eric.auger@redhat.com;
> peter.maydell@linaro.org; nicolinc@nvidia.com; ddutile@redhat.com;
> Linuxarm <linuxarm@huawei.com>; Wangzhou (B)
> <wangzhou1@hisilicon.com>; jiangkunkun <jiangkunkun@huawei.com>;
> Jonathan Cameron <jonathan.cameron@huawei.com>;
> zhangfei.gao@linaro.org
> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
> nested SMMUv3
> 
> 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" ?

Agree. There were similar previous comments from reviewers that current smmuv3 
already has emulated stage 1 and stage 2 support and refers to that as "nested"
in code. So this will be renamed as above. 

Thanks,
Shameer