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

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Feb 07, 2025 at 12:21:54PM +0000, Shameerali Kolothum Thodi wrote:

> Just wondering whether we can have something like the
> vfio_register_iommu_driver() for iommufd subsystem by which it can directly
> access iommu drivers ops(may be a restricted set). 

I very much want to try hard to avoid that.

AFAICT you do not need a VFIO device, or access to the HW_INFO of the
smmu to start up a SMMU driver.

Yes, you cannot later attach a VFIO device with a pSMMU that
materially differs from vSMMU setup, but that is fine.

qemu has long had a duality where you can either "inherit from host"
for an easy setup or be "fully specified" and support live
migration/etc. CPUID as a simple example.

So, what the smmu patches are doing now is "inherit from host" and
that requires a VFIO device to work. I think that is fine.

If you want to do full hotplug then you need to "fully specified" on
the command line so a working vSMMU can be shown to the guest with no
devices, and no kernel involvement.

Obviously this is a highly advanced operating mode as things like IIDR
and errata need to be considered, but I would guess booting with no
vPCI devices is already abnormal.

Jason