linux-kernel - Re: [PATCH v2 3/5] iommu/vt-d: Extend dmar

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Date:   Mon, 20 Mar 2023 10:54:16 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Yi Liu <yi.l.liu@...el.com>
Cc:     joro@...tes.org, alex.williamson@...hat.com, kevin.tian@...el.com,
        robin.murphy@....com, baolu.lu@...ux.intel.com, cohuck@...hat.com,
        eric.auger@...hat.com, nicolinc@...dia.com, kvm@...r.kernel.org,
        mjrosato@...ux.ibm.com, chao.p.peng@...ux.intel.com,
        yi.y.sun@...ux.intel.com, peterx@...hat.com, jasowang@...hat.com,
        shameerali.kolothum.thodi@...wei.com, lulu@...hat.com,
        suravee.suthikulpanit@....com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 3/5] iommu/vt-d: Extend dmar_domain to support nested
 domain

On Thu, Mar 09, 2023 at 12:22:05AM -0800, Yi Liu wrote:
> From: Lu Baolu <baolu.lu@...ux.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@...ux.intel.com>
> Signed-off-by: Yi Liu <yi.l.liu@...el.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