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

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 <redacted>
---
 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