Re: [PATCH v2 03/14] vfio/iommufd: Create nesting parent hwpt with IOMMU_HWPT_ALLOC_PASID flag

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Mar 27, 2026 at 02:29:20AM +0000, Duan, Zhenzhong wrote:
> >-----Original Message-----
> >From: Nicolin Chen <nicolinc@nvidia.com>
> >Subject: Re: [PATCH v2 03/14] vfio/iommufd: Create nesting parent hwpt with
> >IOMMU_HWPT_ALLOC_PASID flag
> >
> >On Thu, Mar 26, 2026 at 05:11:17AM -0400, Zhenzhong Duan wrote:
> >> @@ -430,6 +431,11 @@ static bool
> >iommufd_cdev_autodomains_get(VFIODevice *vbasedev,
> >>          }
> >>      }
> >>
> >> +    if (max_pasid_log2 &&
> >> +        vfio_device_get_viommu_flags_pasid_supported(vbasedev)) {
> >> +        flags |= IOMMU_HWPT_ALLOC_PASID;
> >> +    }
> >
> >This would set it to:
> >	IOMMU_HWPT_ALLOC_PASID | IOMMU_HWPT_ALLOC_NEST_PARENT
> >which isn't supported on ARM :-/
> 
> I am a bit confused, if smmu supports dirty tracking, flags would be
> set to IOMMU_HWPT_ALLOC_DIRTY_TRACKING | IOMMU_HWPT_ALLOC_NEST_PARENT,
> in arm_smmu_domain_alloc_paging_flags(), I see it will return -EOPNOTSUPP.
> So how did smmu work in this case?

You hit a point. I almost forgot we need to do something with that
dirty tracking flag. This is currently broken..

For NVIDIA, the current generation Grace CPU doesn't support dirty
tracking. So, our QEMU VMs don't set that flag. This is just lucky
for us. Yet, it would trigger -EOPNOTSUPP on ARM CPU that supports,
as you mentioned.

For pasid attachment however, ARM doesn't need it: regular pasid=0
attach already has the pointer to a stage-1 PASID table.

Nicolin