RE: [PATCH v2 13/15] vfio/ccw: Use the new device life cycle helpers

[Tian, Kevin]

ping @Eric Farman.

ccw is the only tricky player in this series. Please help take a look in case of
any oversight here.

> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Sent: Thursday, September 1, 2022 10:38 PM
> 
> ccw is the only exception which cannot use vfio_alloc_device() because
> its private device structure is designed to serve both mdev and parent.
> Life cycle of the parent is managed by css_driver so vfio_ccw_private
> must be allocated/freed in css_driver probe/remove path instead of
> conforming to vfio core life cycle for mdev.
> 
> Given that use a wait/completion scheme so the mdev remove path waits
> after vfio_put_device() until receiving a completion notification from
> @release. The completion indicates that all active references on
> vfio_device have been released.
> 
> After that point although free of vfio_ccw_private is delayed to
> css_driver it's at least guaranteed to have no parallel reference on
> released vfio device part from other code paths.
> 
> memset() in @probe is removed. vfio_device is either already cleared
> when probed for the first time or cleared in @release from last probe.
> 
> The right fix is to introduce separate structures for mdev and parent,
> but this won't happen in short term per prior discussions.
> 
> Remove vfio_init/uninit_group_dev() as no user now.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> ---
>  drivers/s390/cio/vfio_ccw_ops.c     | 52 +++++++++++++++++++++++++----
>  drivers/s390/cio/vfio_ccw_private.h |  3 ++
>  drivers/vfio/vfio_main.c            | 23 +++----------
>  include/linux/vfio.h                |  3 --
>  4 files changed, 53 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/s390/cio/vfio_ccw_ops.c
> b/drivers/s390/cio/vfio_ccw_ops.c
> index 4a806a2273b5..9f8486c0d3d3 100644
> --- a/drivers/s390/cio/vfio_ccw_ops.c
> +++ b/drivers/s390/cio/vfio_ccw_ops.c
> @@ -87,6 +87,15 @@ static struct attribute_group *mdev_type_groups[] = {
>  	NULL,
>  };
> 
> +static int vfio_ccw_mdev_init_dev(struct vfio_device *vdev)
> +{
> +	struct vfio_ccw_private *private =
> +		container_of(vdev, struct vfio_ccw_private, vdev);
> +
> +	init_completion(&private->release_comp);
> +	return 0;
> +}
> +
>  static int vfio_ccw_mdev_probe(struct mdev_device *mdev)
>  {
>  	struct vfio_ccw_private *private = dev_get_drvdata(mdev-
> >dev.parent);
> @@ -98,9 +107,9 @@ static int vfio_ccw_mdev_probe(struct mdev_device
> *mdev)
>  	if (atomic_dec_if_positive(&private->avail) < 0)
>  		return -EPERM;
> 
> -	memset(&private->vdev, 0, sizeof(private->vdev));
> -	vfio_init_group_dev(&private->vdev, &mdev->dev,
> -			    &vfio_ccw_dev_ops);
> +	ret = vfio_init_device(&private->vdev, &mdev->dev,
> &vfio_ccw_dev_ops);
> +	if (ret)
> +		return ret;
> 
>  	VFIO_CCW_MSG_EVENT(2, "sch %x.%x.%04x: create\n",
>  			   private->sch->schid.cssid,
> @@ -109,16 +118,33 @@ static int vfio_ccw_mdev_probe(struct
> mdev_device *mdev)
> 
>  	ret = vfio_register_emulated_iommu_dev(&private->vdev);
>  	if (ret)
> -		goto err_atomic;
> +		goto err_put_vdev;
>  	dev_set_drvdata(&mdev->dev, private);
>  	return 0;
> 
> -err_atomic:
> -	vfio_uninit_group_dev(&private->vdev);
> +err_put_vdev:
> +	vfio_put_device(&private->vdev);
>  	atomic_inc(&private->avail);
>  	return ret;
>  }
> 
> +static void vfio_ccw_mdev_release_dev(struct vfio_device *vdev)
> +{
> +	struct vfio_ccw_private *private =
> +		container_of(vdev, struct vfio_ccw_private, vdev);
> +
> +	/*
> +	 * We cannot free vfio_ccw_private here because it includes
> +	 * parent info which must be free'ed by css driver.
> +	 *
> +	 * Use a workaround by memset'ing the core device part and
> +	 * then notifying the remove path that all active references
> +	 * to this device have been released.
> +	 */
> +	memset(vdev, 0, sizeof(*vdev));
> +	complete(&private->release_comp);
> +}
> +
>  static void vfio_ccw_mdev_remove(struct mdev_device *mdev)
>  {
>  	struct vfio_ccw_private *private = dev_get_drvdata(mdev-
> >dev.parent);
> @@ -130,7 +156,17 @@ static void vfio_ccw_mdev_remove(struct
> mdev_device *mdev)
> 
>  	vfio_unregister_group_dev(&private->vdev);
> 
> -	vfio_uninit_group_dev(&private->vdev);
> +	vfio_put_device(&private->vdev);
> +	/*
> +	 * Wait for all active references on mdev are released so it
> +	 * is safe to defer kfree() to a later point.
> +	 *
> +	 * TODO: the clean fix is to split parent/mdev info from ccw
> +	 * private structure so each can be managed in its own life
> +	 * cycle.
> +	 */
> +	wait_for_completion(&private->release_comp);
> +
>  	atomic_inc(&private->avail);
>  }
> 
> @@ -592,6 +628,8 @@ static void vfio_ccw_mdev_request(struct vfio_device
> *vdev, unsigned int count)
>  }
> 
>  static const struct vfio_device_ops vfio_ccw_dev_ops = {
> +	.init = vfio_ccw_mdev_init_dev,
> +	.release = vfio_ccw_mdev_release_dev,
>  	.open_device = vfio_ccw_mdev_open_device,
>  	.close_device = vfio_ccw_mdev_close_device,
>  	.read = vfio_ccw_mdev_read,
> diff --git a/drivers/s390/cio/vfio_ccw_private.h
> b/drivers/s390/cio/vfio_ccw_private.h
> index cd24b7fada91..63d9202b29c7 100644
> --- a/drivers/s390/cio/vfio_ccw_private.h
> +++ b/drivers/s390/cio/vfio_ccw_private.h
> @@ -88,6 +88,7 @@ struct vfio_ccw_crw {
>   * @req_trigger: eventfd ctx for signaling userspace to return device
>   * @io_work: work for deferral process of I/O handling
>   * @crw_work: work for deferral process of CRW handling
> + * @release_comp: synchronization helper for vfio device release
>   */
>  struct vfio_ccw_private {
>  	struct vfio_device vdev;
> @@ -113,6 +114,8 @@ struct vfio_ccw_private {
>  	struct eventfd_ctx	*req_trigger;
>  	struct work_struct	io_work;
>  	struct work_struct	crw_work;
> +
> +	struct completion	release_comp;
>  } __aligned(8);
> 
>  int vfio_ccw_sch_quiesce(struct subchannel *sch);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index c9d982131265..957d9f286550 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -481,28 +481,13 @@ static struct vfio_device
> *vfio_group_get_device(struct vfio_group *group,
>  /*
>   * VFIO driver API
>   */
> -void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
> -			 const struct vfio_device_ops *ops)
> -{
> -	init_completion(&device->comp);
> -	device->dev = dev;
> -	device->ops = ops;
> -}
> -EXPORT_SYMBOL_GPL(vfio_init_group_dev);
> -
> -void vfio_uninit_group_dev(struct vfio_device *device)
> -{
> -	vfio_release_device_set(device);
> -}
> -EXPORT_SYMBOL_GPL(vfio_uninit_group_dev);
> -
>  /* Release helper called by vfio_put_device() */
>  void vfio_device_release(struct kref *kref)
>  {
>  	struct vfio_device *device =
>  			container_of(kref, struct vfio_device, kref);
> 
> -	vfio_uninit_group_dev(device);
> +	vfio_release_device_set(device);
> 
>  	/*
>  	 * kvfree() cannot be done here due to a life cycle mess in
> @@ -560,7 +545,9 @@ int vfio_init_device(struct vfio_device *device, struct
> device *dev,
>  {
>  	int ret;
> 
> -	vfio_init_group_dev(device, dev, ops);
> +	init_completion(&device->comp);
> +	device->dev = dev;
> +	device->ops = ops;
> 
>  	if (ops->init) {
>  		ret = ops->init(device);
> @@ -572,7 +559,7 @@ int vfio_init_device(struct vfio_device *device, struct
> device *dev,
>  	return 0;
> 
>  out_uninit:
> -	vfio_uninit_group_dev(device);
> +	vfio_release_device_set(device);
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(vfio_init_device);
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e1e9e8352903..f03447c8774d 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -160,9 +160,6 @@ static inline void vfio_put_device(struct vfio_device
> *device)
>  	kref_put(&device->kref, vfio_device_release);
>  }
> 
> -void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
> -			 const struct vfio_device_ops *ops);
> -void vfio_uninit_group_dev(struct vfio_device *device);
>  int vfio_register_group_dev(struct vfio_device *device);
>  int vfio_register_emulated_iommu_dev(struct vfio_device *device);
>  void vfio_unregister_group_dev(struct vfio_device *device);
> --
> 2.21.3