Re: [RFC PATCH v6 05/11] iommu: Add a helper to validate a vIOMMU parent

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Jason Gunthorpe <jgg@nvidia.com> writes:

> On Mon, Sep 28, 2026 at 04:06:42PM +0530, Aneesh Kumar K.V wrote:
>> Jason Gunthorpe <jgg@nvidia.com> writes:
>> 
>> > On Fri, Sep 25, 2026 at 11:18:44AM +0530, Aneesh Kumar K.V wrote:
>> >> > The TSM viommu should use a NULL parent domain, it doesn't have an
>> >> > iommufd managed S2.
>> >> 
>> >> How would we assign an untrusted device? I currently follow these steps:
>> >
>> >> 1. Create an HWPT with IOMMU_HWPT_ALLOC_NEST_PARENT.
>> >> 2. Allocate a vIOMMU with viommu.hwpt_id set to that hwpt_id.
>> >> 3. Allocate a vdevice with alloc_vdev.viommu_id set to that viommu_id.
>> >> 4. Use VFIO_DEVICE_ATTACH_IOMMUFD_PT with the hwpt_id.
>> >
>> > The vmiommu.hwpt_id should be 0.
>> >
>> > 1. Create a a HWPT with IOMMU_HWPT_ALLOC_NEST_PARENT
>> > 2. VFIO_DEVICE_ATTACH_IOMMUFD_PT with the hwpt_id to establish the T=0
>> >    identity S2, no T=0 vSMMU
>> > 3. Create a VIOMMU with no hwpt_id and the RMM's type. This triggers
>> >    RMM to create the the T=1 vSMMU inside the realm
>> > 4. Allocate a vdevice on the viommu_id. This triggers RMM to create
>> >    the VDEV inside the realm
>> > 5. Setup guest ACPI tables/etc to point at the RMM's T=1 vsmmu.
>> >
>> > Now both the T=1 VSUMM and T=0 fixed translation are setup.
>> >
>> 
>> ok so iommufd_viommu_alloc_ioctl() will do this based on type.
>> 
>> +	if (iommufd_viommu_type_requires_hwpt(cmd->type)) {
>> +		hwpt_paging = iommufd_get_hwpt_paging(ucmd, cmd->hwpt_id);
>> +		if (IS_ERR(hwpt_paging)) {
>
> The core code should not decode type, that's always a hack..
>
> The ideal thing is to order things so we get a viommu_ops before
> trying to get the hwpt, then use a flag in the ops
>

Does that mean the SMMU driver will return viommu_ops before
iommu_ops->viommu_init() is called? If so, should viommu_init() be moved
into viommu_ops?

Would the core then only need to do something like:

	rc = tsm_viommu_get_info(idev->dev, cmd->type, &info);
	if (not_using_tsm) // All smmu driver will now support get_viommu_info.
		rc = iommu_ops->get_viommu_info(idev->dev, cmd->type, &info);

and then call info->ops->viommu_init()?

-aneesh