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

Nicolin Chen
2025-04-26T06:19:56Z

NVIDIA Virtual Command Queue is one of the iommufd users exposing vIOMMU
features to user space VMs. Its hardware has a strict rule when mapping
and unmapping multiple global CMDQVs to/from a VM-owned VINTF, requiring
mappings in ascending order and unmappings in descending order.

The tegra241-cmdqv driver can apply the rule for a mapping in the LVCMDQ
allocation handler, however it can't do the same for an unmapping since
the destroy op returns void.

Add iommufd_vcmdq_depend/undepend() for-driver helpers, allowing LVCMDQ
allocator to refcount_inc() a sibling LVCMDQ object and LVCMDQ destroyer
to refcount_dec().

This is a bit of compromise, because a driver might end up with abusing
the API that deadlocks the objects. So restrict the API to a dependency
between two driver-allocated objects of the same type, as iommufd would
unlikely build any core-level dependency in this case.

Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
include/linux/iommufd.h | 47 ++++++++++++++++++++++++++++++++++
drivers/iommu/iommufd/driver.c | 28 ++++++++++++++++++++
2 files changed, 75 insertions(+)

diff --git a/include/linux/iommufd.h b/include/linux/iommufd.h
index e91381aaec5a..5dff154e8ce1 100644
--- a/include/linux/iommufd.h
+++ b/include/linux/iommufd.h
@@ -232,6 +232,10 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
size_t size,
enum iommufd_object_type type);
void iommufd_object_abort(struct iommufd_ctx *ictx, struct iommufd_object *obj);
+int iommufd_object_depend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended);
+void iommufd_object_undepend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended);
struct device *iommufd_viommu_find_dev(struct iommufd_viommu *viommu,
unsigned long vdev_id);
int iommufd_viommu_get_vdev_id(struct iommufd_viommu *viommu,
@@ -252,6 +256,17 @@ static inline void iommufd_object_abort(struct iommufd_ctx *ictx,
{
}

+static inline int iommufd_object_depend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended)
+{
+ return -EOPNOTSUPP;
+}
+
+static inline void iommufd_object_undepend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended)
+{
+}
+
static inline struct device *
iommufd_viommu_find_dev(struct iommufd_viommu *viommu, unsigned long vdev_id)
{
@@ -329,4 +344,36 @@ static inline int iommufd_viommu_report_event(struct iommufd_viommu *viommu,
static_assert(offsetof(typeof(*drv_struct), member.obj) == 0); \
iommufd_object_abort(ictx, &drv_struct->member.obj); \
})
+
+/*
+ * Helpers for IOMMU driver to build/destroy a dependency between two sibling
+ * structures created by one of the allocators above
+ */
+#define iommufd_vcmdq_depend(vcmdq_dependent, vcmdq_depended, member) \
+ ({ \
+ static_assert(__same_type(struct iommufd_object, \
+ vcmdq_dependent->member.obj)); \
+ static_assert(offsetof(typeof(*vcmdq_dependent), \
+ member.obj) == 0); \
+ static_assert(__same_type(struct iommufd_object, \
+ vcmdq_depended->member.obj)); \
+ static_assert(offsetof(typeof(*vcmdq_depended), \
+ member.obj) == 0); \
+ iommufd_object_depend(&vcmdq_dependent->member.obj, \
+ &vcmdq_depended->member.obj); \
+ })
+
+#define iommufd_vcmdq_undepend(vcmdq_dependent, vcmdq_depended, member) \
+ ({ \
+ static_assert(__same_type(struct iommufd_object, \
+ vcmdq_dependent->member.obj)); \
+ static_assert(offsetof(typeof(*vcmdq_dependent), \
+ member.obj) == 0); \
+ static_assert(__same_type(struct iommufd_object, \
+ vcmdq_depended->member.obj)); \
+ static_assert(offsetof(typeof(*vcmdq_depended), \
+ member.obj) == 0); \
+ iommufd_object_undepend(&vcmdq_dependent->member.obj, \
+ &vcmdq_depended->member.obj); \
+ })
#endif
diff --git a/drivers/iommu/iommufd/driver.c b/drivers/iommu/iommufd/driver.c
index 7980a09761c2..fb7f8fe40f95 100644
--- a/drivers/iommu/iommufd/driver.c
+++ b/drivers/iommu/iommufd/driver.c
@@ -50,6 +50,34 @@ void iommufd_object_abort(struct iommufd_ctx *ictx, struct iommufd_object *obj)
}
EXPORT_SYMBOL_NS_GPL(iommufd_object_abort, "IOMMUFD");

+/* A per-structure helper is available in include/linux/iommufd.h */
+int iommufd_object_depend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended)
+{
+ /* Reject self dependency that dead locks */
+ if (obj_dependent == obj_depended)
+ return -EINVAL;
+ /* Only support dependency between two objects of the same type */
+ if (obj_dependent->type != obj_depended->type)
+ return -EINVAL;
+
+ refcount_inc(&obj_depended->users);
+ return 0;
+}
+EXPORT_SYMBOL_NS_GPL(iommufd_object_depend, "IOMMUFD");
+
+/* A per-structure helper is available in include/linux/iommufd.h */
+void iommufd_object_undepend(struct iommufd_object *obj_dependent,
+ struct iommufd_object *obj_depended)
+{
+ if (WARN_ON_ONCE(obj_dependent == obj_depended ||
+ obj_dependent->type != obj_depended->type))
+ return;
+
+ refcount_dec(&obj_depended->users);
+}
+EXPORT_SYMBOL_NS_GPL(iommufd_object_undepend, "IOMMUFD");
+
/* 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)
--
2.43.0