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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, February 6, 2025 6:13 PM
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: Daniel P. Berrangé <berrange@redhat.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; nathanc@nvidia.com
> Subject: Re: [RFC PATCH 0/5] hw/arm/virt: Add support for user-creatable
> nested SMMUv3
> 
> 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.

It is not. Qemu just instantiates a vSMMU and assigns the IOMMU 
instance id to it.

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

Creating the vIOMMU only happens when the user does a  cold/hot plug of
a VFIO device. At that time Qemu checks whether the assigned id matches
with whatever the kernel tell it. 

Thanks,
Shameer