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

On Sun, Sep 01, 2024 at 10:39:17AM +0800, Baolu Lu wrote:

On 2024/8/28 0:59, Nicolin Chen wrote:

quoted

+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