Re: [PATCH v2 3/5] iommu/vt-d: Extend dmar_domain to support nested domain

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Mar 09, 2023 at 12:22:05AM -0800, Yi Liu wrote:
> From: Lu Baolu <baolu.lu@linux.intel.com>
> 
> The nested domain fields are exclusive to those that used for a DMA
> remapping domain. Use union to avoid memory waste.
> 
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> ---
>  drivers/iommu/intel/iommu.h | 35 +++++++++++++++++++++++++++++------
>  1 file changed, 29 insertions(+), 6 deletions(-)

Using unions like this often devolves into a mess.

You'd be better to have more structures

struct intel_iommu_domain {
   struct iommu_domain domain;
   [general fields about attachment]
};

struct intel_iopte_domain {
    struct intel_iommu_domain domain;
    [stuff describing the io page table data, pgd, format, etc]
};

strut intel_s1_domain {
     struct intel_iommu_domain domain;
     struct dmar_domain *s2_domain;
     /* user page table pointer (in GPA) */
     unsigned long s1_pgtbl;
     /* page table attributes */
     struct iommu_hwpt_intel_vtd s1_cfg;
};
static_assert(offset_of(struct intel_s1_domain, domain.domain) ==
              offset_of(struct intel_iommu_domain, domain));

The per-domain ops allow to make this work sensibly

Jason