Re: [PATCH v2 11/22] iommufd: Add for-driver helpers iommufd_vcmdq_depend/undepend()

Nicolin Chen
2025-04-29T17:13:01Z

On Tue, Apr 29, 2025 at 12:40:07PM +0000, Pranjal Shrivastava wrote:
> On Fri, Apr 25, 2025 at 10:58:06PM -0700, Nicolin Chen wrote:
> > /* Caller should xa_lock(&viommu->vdevs) to protect the return value */
> > struct device *iommufd_viommu_find_dev(struct iommufd_viommu *viommu,
> > unsigned long vdev_id)
>
> If I'm getting this right, I think we are setting up dependencies like:
> vcmdq[2] -> vcmdq[1] -> vcmdq[0] based on refcounts of each object,
> which ensures that the unmaps happen in descending order..

Yes.

> If that's right, Is it fair to have iommufd_vcmdq_depend/undepend in the
> core code itself? Since it's a driver-level limitation, I think we
> should just have iommufd_object_depend/undepend in the core code and the
> iommufd_vcmdq_depend/undepend can move into the CMDQV driver?

The moment we added iommufd_object_depend/undepend, we already had
a blur boundary here since we had no choice to handle in the driver
but to ask core for help.

The iommufd_vcmdq_depend/undepend is just a pair of macros to help
validating the structure inputs that are core defined. It is quite
fair to put next to the raw functions. I also had the notes on top
of the raw functions suggesting callers to use the macros instead.

Thanks
Nicolin