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

[Pranjal Shrivastava <praan@google.com>]

On Tue, Apr 29, 2025 at 11:07:42AM -0700, Nicolin Chen wrote:
> On Tue, Apr 29, 2025 at 05:59:32PM +0000, Pranjal Shrivastava wrote:
> > On Tue, Apr 29, 2025 at 10:10:28AM -0700, Nicolin Chen wrote:
> > > 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.
> > > 
> > 
> > Well, yes.. in that case let's call the macros something else? The
> > current names suggest that the macros only setup dependencies for vcmdq
> > and not any "two sibling structures created by one of the allocators
> > above" as mentioned by the note. Maybe we could rename the macro to
> > something like: `iommufd_container_obj_depend`?
> 
> That's the intention of the macros: to validate vCMDQ structure
> and help covert a driver-defined vcmdq structure to the required
> core field, as we only have vCMDQ objects using them.
> 
> If we have use case for other objects in the future, we should
> add another iommufd_vxxxx_depend/undepend macros.

Thanks for clarifying the rationale behind the VCMDQ-specific naming.

On the point of needing new iommufd_vxxxx_depend macros for future
object types, I don't think that would be required because the current
static_asserts within these macros validate the container->member.obj
embedding pattern, not the struct type of the container itself which
makes the macro logic inherently reusable for any other object type 
that adopts the same embedding.

However, if there's  a strong preference against making it generic,
I don't have any issues since we only use it for vCMDQs right now.

My main point was to keep the core code seem generic to aid other
implementations in the future... today NVIDIA has CMDQV, tomorrow maybe
someone else would have something for vdevice or something. Anyway, I
don't feel strongly about this. Just trying to help :)

> 
> Nicolin

Thanks,
Praan