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

[Jason Gunthorpe]

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 at 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.

> +/*
> + * 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

Jason