Re: [PATCH v8 13/15] iommufd/viommu: Allow vIOMMUs without a parent HWPT

[Jason Gunthorpe]

On Wed, Nov 04, 2026 at 11:03:41AM -0800, Nicolin Chen wrote:
> On Wed, Nov 04, 2026 at 09:46:30PM +0530, Aneesh Kumar K.V (Arm) wrote:
> > VIOMMU_ALLOC currently requires a nesting parent HWPT_PAGING even when
> > the selected vIOMMU implementation has no use for one. This would force
> > implementations such as the following Arm Realm vIOMMU to create an
> > unused parent.
> 
> IIRC, MSI on ARM still goes through the NS pathway. So, it wouldn't
> be "unused" if the device attaches to it (or its nested hwpts)?

It is unused by the RMM VIOMMU. I think that is what is Aneesh is
explaining.

> By dropping the nesting parent from VIOMMU, it just simplifies the
> attachment to an S1 paging hwpt, right?

Yes.

The VMM must always maintain a S1 paging attachment that is the
identity map of the VM on all devices, same action as when there is no
viommu.

Jason