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

Jason Gunthorpe
2024-09-04T16:30:56Z

On Sun, Sep 01, 2024 at 10:27:09PM -0700, Nicolin Chen wrote:
> 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.

Eventually you need a way to pin the physical iommu, without pinning
any idevs. Not sure how best to do that

Jason