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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Feb 06, 2025 at 06:04:57PM +0000, Shameerali Kolothum Thodi wrote:
> > Some kind of hot plug smmu would have to create a vSMMU without any
> > kernel backing and then later bind it to a kernel implementation.
> 
> Not sure I get the problem with associating vSMMU with a pSMMU. Something
> like an iommu instance id mentioned before,
> 
> -device arm-smmuv3-accel,id=smmuv2,bus=pcie.2,host-smmu=iommu.1
> 
> This can realize the vSMMU without actually creating a vIOMMU in kernel.
> And when the dev gets attached/realized, check (GET_HW_INFO)the specified
> iommu instance id matches or not.
> 
> Or the concern here is exporting an iommu instance id to user space?

Philisophically we do not permit any HW access through iommufd without
a VFIO fd to "prove" the process has rights to touch hardware.

We don't have any way to prove the process has rights to touch the
iommu hardware seperately from VFIO.

So even if you invent an iommu ID we cannot accept it as a handle to
create viommu in iommufd.

Jason