Re: [PATCH v2 10/19] iommufd/viommu: Add vdev_id helpers for IOMMU drivers

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Sep 05, 2024 at 01:14:15PM -0300, Jason Gunthorpe wrote:
> On Tue, Aug 27, 2024 at 09:59:47AM -0700, Nicolin Chen wrote:
> > Driver can call the iommufd_viommu_find_device() to find a device pointer
> > using its per-viommu virtual ID. The returned device must be protected by
> > the pair of iommufd_viommu_lock/unlock_vdev_id() function.
> > 
> > Put these three functions into a new viommu_api file, to build it with the
> > IOMMUFD_DRIVER config.
> > 
> > Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> > ---
> >  drivers/iommu/iommufd/Makefile     |  2 +-
> >  drivers/iommu/iommufd/viommu_api.c | 39 ++++++++++++++++++++++++++++++
> >  include/linux/iommufd.h            | 16 ++++++++++++
> >  3 files changed, 56 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/iommu/iommufd/viommu_api.c
> 
> I still think this is better to just share the struct content with the
> driver, eventually we want to do this anyhow as the driver will
> want to use container_of() techniques to reach its private data.

In my mind, exposing everything to the driver is something that
we have to (for driver-managed structures) v.s. we want to...
Even in that case, a driver actually only need to know the size
of the core structure, without touching what's inside(?).

I am a bit worried that drivers would abuse the content in the
core-level structure.. Providing a set of API would encourage
them to keep the core structure intact, hopefully..

> > +/*
> > + * Find a device attached to an VIOMMU object using a virtual device ID that was
> > + * set via an IOMMUFD_CMD_VIOMMU_SET_VDEV_ID. Callers of this function must call
> > + * iommufd_viommu_lock_vdev_id() prior and iommufd_viommu_unlock_vdev_id() after
> > + *
> > + * Return device or NULL.
> > + */
> > +struct device *iommufd_viommu_find_device(struct iommufd_viommu *viommu, u64 id)
> > +{
> > +	struct iommufd_vdev_id *vdev_id;
> > +
> > +	lockdep_assert_held(&viommu->vdev_ids_rwsem);
> > +
> > +	xa_lock(&viommu->vdev_ids);
> > +	vdev_id = xa_load(&viommu->vdev_ids, (unsigned long)id);
> > +	xa_unlock(&viommu->vdev_ids);
> 
> No need for this lock, xa_load is rcu safe against concurrent writer

I see iommufd's device.c and main.c grab xa_lock before xa_load?

Thanks
Nicolin