Re: [PATCH v6 07/25] iommufd/access: Add internal APIs for HW queue to use

[Pranjal Shrivastava <praan@google.com>]

On Sat, Jun 14, 2025 at 12:14:32AM -0700, Nicolin Chen wrote:
> Now, access->ops can be NULL, to support an internal use case for the new
> HW queue object. Since an access object in this case will be allocated by
> an inernal iommufd object, the refcount on the ictx should be skipped, so
> as not to deadlock the release of the ictx as it would otherwise wait for
> the release of the access first during the release of the internal object
> that could wait for the release of ictx:
>     ictx --releases--> hw_queue --releases--> access
>       ^                                         |
>       |_________________releases________________v
> 
> Add a set of lightweight internal APIs to unlink access and ictx:
>     ictx --releases--> hw_queue --releases--> access
> 
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>  drivers/iommu/iommufd/iommufd_private.h |  8 ++++
>  drivers/iommu/iommufd/device.c          | 59 +++++++++++++++++++++----
>  2 files changed, 58 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
> index 4a375a8c9216..468717d5e5bc 100644
> --- a/drivers/iommu/iommufd/iommufd_private.h
> +++ b/drivers/iommu/iommufd/iommufd_private.h
> @@ -484,6 +484,14 @@ void iopt_remove_access(struct io_pagetable *iopt,
>  			struct iommufd_access *access, u32 iopt_access_list_id);
>  void iommufd_access_destroy_object(struct iommufd_object *obj);
>  
> +/* iommufd_access for internal use */
> +struct iommufd_access *iommufd_access_create_internal(struct iommufd_ctx *ictx);
> +#define iommufd_access_destroy_internal(ictx, access) \
> +	iommufd_object_destroy_user(ictx, &(access)->obj)
> +int iommufd_access_attach_internal(struct iommufd_access *access,
> +				   struct iommufd_ioas *ioas);
> +#define iommufd_access_detach_internal(access) iommufd_access_detach(access)
> +
>  struct iommufd_eventq {
>  	struct iommufd_object obj;
>  	struct iommufd_ctx *ictx;
> diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c
> index 9293722b9cff..ad33f1e41a24 100644
> --- a/drivers/iommu/iommufd/device.c
> +++ b/drivers/iommu/iommufd/device.c
> @@ -1084,7 +1084,39 @@ void iommufd_access_destroy_object(struct iommufd_object *obj)
>  	if (access->ioas)
>  		WARN_ON(iommufd_access_change_ioas(access, NULL));
>  	mutex_unlock(&access->ioas_lock);
> -	iommufd_ctx_put(access->ictx);
> +	if (access->ops)
> +		iommufd_ctx_put(access->ictx);

Purely relying on access->ops being NULL feels a bit hacky to me..

> +}
> +
> +static struct iommufd_access *__iommufd_access_create(struct iommufd_ctx *ictx)
> +{
> +	struct iommufd_access *access;
> +
> +	/*
> +	 * There is no uAPI for the access object, but to keep things symmetric
> +	 * use the object infrastructure anyhow.
> +	 */
> +	access = iommufd_object_alloc(ictx, access, IOMMUFD_OBJ_ACCESS);
> +	if (IS_ERR(access))
> +		return access;
> +
> +	/* The calling driver is a user until iommufd_access_destroy() */
> +	refcount_inc(&access->obj.users);
> +	mutex_init(&access->ioas_lock);
> +	return access;
> +}
> +
> +struct iommufd_access *iommufd_access_create_internal(struct iommufd_ctx *ictx)
> +{
> +	struct iommufd_access *access;
> +
> +	access = __iommufd_access_create(ictx);
> +	if (IS_ERR(access))
> +		return access;
> +	access->iova_alignment = PAGE_SIZE;

Maybe setting acces->ictx = NULL; explicitly here would be a clear
demarcation between the new API for "internal" v/s the original one.
Else, I definitely believe we should have a comment mentioning that
access->ictx is NULL for internal.

> +
> +	iommufd_object_finalize(ictx, &access->obj);
> +	return access;
>  }
>  
>  /**
> @@ -1106,11 +1138,7 @@ iommufd_access_create(struct iommufd_ctx *ictx,
>  {
>  	struct iommufd_access *access;
>  
> -	/*
> -	 * There is no uAPI for the access object, but to keep things symmetric
> -	 * use the object infrastructure anyhow.
> -	 */
> -	access = iommufd_object_alloc(ictx, access, IOMMUFD_OBJ_ACCESS);
> +	access = __iommufd_access_create(ictx);
>  	if (IS_ERR(access))
>  		return access;
>  
> @@ -1122,13 +1150,10 @@ iommufd_access_create(struct iommufd_ctx *ictx,
>  	else
>  		access->iova_alignment = 1;
>  
> -	/* The calling driver is a user until iommufd_access_destroy() */
> -	refcount_inc(&access->obj.users);
>  	access->ictx = ictx;
>  	iommufd_ctx_get(ictx);
>  	iommufd_object_finalize(ictx, &access->obj);
>  	*id = access->obj.id;
> -	mutex_init(&access->ioas_lock);
>  	return access;
>  }
>  EXPORT_SYMBOL_NS_GPL(iommufd_access_create, "IOMMUFD");
> @@ -1173,6 +1198,22 @@ int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id)
>  }
>  EXPORT_SYMBOL_NS_GPL(iommufd_access_attach, "IOMMUFD");
>  
> +int iommufd_access_attach_internal(struct iommufd_access *access,
> +				   struct iommufd_ioas *ioas)
> +{
> +	int rc;
> +
> +	mutex_lock(&access->ioas_lock);
> +	if (WARN_ON(access->ioas)) {
> +		mutex_unlock(&access->ioas_lock);
> +		return -EINVAL;
> +	}
> +
> +	rc = iommufd_access_change_ioas(access, ioas);
> +	mutex_unlock(&access->ioas_lock);
> +	return rc;
> +}
> +
>  int iommufd_access_replace(struct iommufd_access *access, u32 ioas_id)
>  {
>  	int rc;
> -- 
> 2.43.0
>