Re: [PATCH v2 02/19] iommufd/viommu: Add IOMMUFD_OBJ_VIOMMU and IOMMU_VIOMMU_ALLOC ioctl

[Nicolin Chen <nicolinc@nvidia.com>]

On Sun, Sep 01, 2024 at 10:39:17AM +0800, Baolu Lu wrote:
> On 2024/8/28 0:59, Nicolin Chen wrote:
> > +int iommufd_viommu_alloc_ioctl(struct iommufd_ucmd *ucmd)
> > +{
> > +     struct iommu_viommu_alloc *cmd = ucmd->cmd;
> > +     struct iommufd_hwpt_paging *hwpt_paging;
> > +     struct iommufd_viommu *viommu;
> > +     struct iommufd_device *idev;
> > +     int rc;
> > +
> > +     if (cmd->flags)
> > +             return -EOPNOTSUPP;
> > +
> > +     idev = iommufd_get_device(ucmd, cmd->dev_id);
> 
> Why does a device reference count is needed here? When is this reference
> count released after the VIOMMU is allocated?

Hmm, it was used to get dev->iommu->iommu_dev to pin the VIOMMU to
a physical IOMMU instance (in v1). Jason suggested to remove that,
yet I didn't realize that this idev is now completely useless.

With that being said, a parent HWPT could be shared across VIOMUs
allocated for the same VM. So, I think we do need a dev pointer to
know which physical instance the VIOMMU allocates for, especially
for a driver-managed VIOMMU.

Perhaps we should add back the iommu_dev and properly refcount it.

Thanks
Nicolin