RE: [PATCH v2 03/14] vfio: Split creation of a vfio_device into init and register ops

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Saturday, March 13, 2021 8:56 AM
> 
> This makes the struct vfio_pci_device part of the public interface so it
> can be used with container_of and so forth, as is typical for a Linux
> subystem.
> 
> This is the first step to bring some type-safety to the vfio interface by
> allowing the replacement of 'void *' and 'struct device *' inputs with a
> simple and clear 'struct vfio_pci_device *'
> 
> For now the self-allocating vfio_add_group_dev() interface is kept so each
> user can be updated as a separate patch.
> 
> The expected usage pattern is
> 
>   driver core probe() function:
>      my_device = kzalloc(sizeof(*mydevice));
>      vfio_init_group_dev(&my_device->vdev, dev, ops, mydevice);
>      /* other driver specific prep */
>      vfio_register_group_dev(&my_device->vdev);
>      dev_set_drvdata(my_device);

dev_set_drvdata(dev, my_device);

> 
>   driver core remove() function:
>      my_device = dev_get_drvdata(dev);
>      vfio_unregister_group_dev(&my_device->vdev);
>      /* other driver specific tear down */
>      kfree(my_device);
> 
> Allowing the driver to be able to use the drvdata and vifo_device to go
> to/from its own data.
> 
> The pattern also makes it clear that vfio_register_group_dev() must be
> last in the sequence, as once it is called the core code can immediately
> start calling ops. The init/register gap is provided to allow for the
> driver to do setup before ops can be called and thus avoid races.
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Liu Yi L <yi.l.liu@intel.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  Documentation/driver-api/vfio.rst |  31 ++++----
>  drivers/vfio/vfio.c               | 123 ++++++++++++++++--------------
>  include/linux/vfio.h              |  16 ++++
>  3 files changed, 98 insertions(+), 72 deletions(-)
> 
> diff --git a/Documentation/driver-api/vfio.rst b/Documentation/driver-
> api/vfio.rst
> index f1a4d3c3ba0bb1..d3a02300913a7f 100644
> --- a/Documentation/driver-api/vfio.rst
> +++ b/Documentation/driver-api/vfio.rst
> @@ -249,18 +249,23 @@ VFIO bus driver API
> 
>  VFIO bus drivers, such as vfio-pci make use of only a few interfaces
>  into VFIO core.  When devices are bound and unbound to the driver,
> -the driver should call vfio_add_group_dev() and vfio_del_group_dev()
> -respectively::
> -
> -	extern int vfio_add_group_dev(struct device *dev,
> -				      const struct vfio_device_ops *ops,
> -				      void *device_data);
> -
> -	extern void *vfio_del_group_dev(struct device *dev);
> -
> -vfio_add_group_dev() indicates to the core to begin tracking the
> -iommu_group of the specified dev and register the dev as owned by
> -a VFIO bus driver.  The driver provides an ops structure for callbacks
> +the driver should call vfio_register_group_dev() and
> +vfio_unregister_group_dev() respectively::
> +
> +	void vfio_init_group_dev(struct vfio_device *device,
> +				struct device *dev,
> +				const struct vfio_device_ops *ops,
> +				void *device_data);
> +	int vfio_register_group_dev(struct vfio_device *device);
> +	void vfio_unregister_group_dev(struct vfio_device *device);
> +
> +The driver should embed the vfio_device in its own structure and call
> +vfio_init_group_dev() to pre-configure it before going to registration.
> +vfio_register_group_dev() indicates to the core to begin tracking the
> +iommu_group of the specified dev and register the dev as owned by a VFIO
> bus
> +driver. Once vfio_register_group_dev() returns it is possible for userspace
> to
> +start accessing the driver, thus the driver should ensure it is completely
> +ready before calling it. The driver provides an ops structure for callbacks
>  similar to a file operations structure::
> 
>  	struct vfio_device_ops {
> @@ -276,7 +281,7 @@ similar to a file operations structure::
>  	};
> 
>  Each function is passed the device_data that was originally registered
> -in the vfio_add_group_dev() call above.  This allows the bus driver
> +in the vfio_register_group_dev() call above.  This allows the bus driver
>  an easy place to store its opaque, private data.  The open/release
>  callbacks are issued when a new file descriptor is created for a
>  device (via VFIO_GROUP_GET_DEVICE_FD).  The ioctl interface provides
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 32660e8a69ae20..cfa06ae3b9018b 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -89,16 +89,6 @@ struct vfio_group {
>  	struct blocking_notifier_head	notifier;
>  };
> 
> -struct vfio_device {
> -	refcount_t			refcount;
> -	struct completion		comp;
> -	struct device			*dev;
> -	const struct vfio_device_ops	*ops;
> -	struct vfio_group		*group;
> -	struct list_head		group_next;
> -	void				*device_data;
> -};
> -
>  #ifdef CONFIG_VFIO_NOIOMMU
>  static bool noiommu __read_mostly;
>  module_param_named(enable_unsafe_noiommu_mode,
> @@ -532,35 +522,6 @@ static struct vfio_group
> *vfio_group_get_from_dev(struct device *dev)
>  /**
>   * Device objects - create, release, get, put, search
>   */
> -static
> -struct vfio_device *vfio_group_create_device(struct vfio_group *group,
> -					     struct device *dev,
> -					     const struct vfio_device_ops *ops,
> -					     void *device_data)
> -{
> -	struct vfio_device *device;
> -
> -	device = kzalloc(sizeof(*device), GFP_KERNEL);
> -	if (!device)
> -		return ERR_PTR(-ENOMEM);
> -
> -	refcount_set(&device->refcount, 1);
> -	init_completion(&device->comp);
> -	device->dev = dev;
> -	/* Our reference on group is moved to the device */
> -	device->group = group;
> -	device->ops = ops;
> -	device->device_data = device_data;
> -	dev_set_drvdata(dev, device);
> -
> -	mutex_lock(&group->device_lock);
> -	list_add(&device->group_next, &group->device_list);
> -	group->dev_counter++;
> -	mutex_unlock(&group->device_lock);
> -
> -	return device;
> -}
> -
>  /* Device reference always implies a group reference */
>  void vfio_device_put(struct vfio_device *device)
>  {
> @@ -779,14 +740,23 @@ static int vfio_iommu_group_notifier(struct
> notifier_block *nb,
>  /**
>   * VFIO driver API
>   */
> -int vfio_add_group_dev(struct device *dev,
> -		       const struct vfio_device_ops *ops, void *device_data)
> +void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
> +			 const struct vfio_device_ops *ops, void *device_data)
> +{
> +	init_completion(&device->comp);
> +	device->dev = dev;
> +	device->ops = ops;
> +	device->device_data = device_data;
> +}
> +EXPORT_SYMBOL_GPL(vfio_init_group_dev);
> +
> +int vfio_register_group_dev(struct vfio_device *device)
>  {
> +	struct vfio_device *existing_device;
>  	struct iommu_group *iommu_group;
>  	struct vfio_group *group;
> -	struct vfio_device *device;
> 
> -	iommu_group = iommu_group_get(dev);
> +	iommu_group = iommu_group_get(device->dev);
>  	if (!iommu_group)
>  		return -EINVAL;
> 
> @@ -805,21 +775,50 @@ int vfio_add_group_dev(struct device *dev,
>  		iommu_group_put(iommu_group);
>  	}
> 
> -	device = vfio_group_get_device(group, dev);
> -	if (device) {
> -		dev_WARN(dev, "Device already exists on group %d\n",
> +	existing_device = vfio_group_get_device(group, device->dev);
> +	if (existing_device) {
> +		dev_WARN(device->dev, "Device already exists on
> group %d\n",
>  			 iommu_group_id(iommu_group));
> -		vfio_device_put(device);
> +		vfio_device_put(existing_device);
>  		vfio_group_put(group);
>  		return -EBUSY;
>  	}
> 
> -	device = vfio_group_create_device(group, dev, ops, device_data);
> -	if (IS_ERR(device)) {
> -		vfio_group_put(group);
> -		return PTR_ERR(device);
> -	}
> +	/* Our reference on group is moved to the device */
> +	device->group = group;
> +
> +	/* Refcounting can't start until the driver calls register */
> +	refcount_set(&device->refcount, 1);
> +
> +	mutex_lock(&group->device_lock);
> +	list_add(&device->group_next, &group->device_list);
> +	group->dev_counter++;
> +	mutex_unlock(&group->device_lock);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(vfio_register_group_dev);
> +
> +int vfio_add_group_dev(struct device *dev, const struct vfio_device_ops
> *ops,
> +		       void *device_data)
> +{
> +	struct vfio_device *device;
> +	int ret;
> +
> +	device = kzalloc(sizeof(*device), GFP_KERNEL);
> +	if (!device)
> +		return -ENOMEM;
> +
> +	vfio_init_group_dev(device, dev, ops, device_data);
> +	ret = vfio_register_group_dev(device);
> +	if (ret)
> +		goto err_kfree;
> +	dev_set_drvdata(dev, device);
>  	return 0;
> +
> +err_kfree:
> +	kfree(device);
> +	return ret;
>  }
>  EXPORT_SYMBOL_GPL(vfio_add_group_dev);
> 
> @@ -887,11 +886,9 @@ EXPORT_SYMBOL_GPL(vfio_device_data);
>  /*
>   * Decrement the device reference count and wait for the device to be
>   * removed.  Open file descriptors for the device... */
> -void *vfio_del_group_dev(struct device *dev)
> +void vfio_unregister_group_dev(struct vfio_device *device)
>  {
> -	struct vfio_device *device = dev_get_drvdata(dev);
>  	struct vfio_group *group = device->group;
> -	void *device_data = device->device_data;
>  	struct vfio_unbound_dev *unbound;
>  	unsigned int i = 0;
>  	bool interrupted = false;
> @@ -908,7 +905,7 @@ void *vfio_del_group_dev(struct device *dev)
>  	 */
>  	unbound = kzalloc(sizeof(*unbound), GFP_KERNEL);
>  	if (unbound) {
> -		unbound->dev = dev;
> +		unbound->dev = device->dev;
>  		mutex_lock(&group->unbound_lock);
>  		list_add(&unbound->unbound_next, &group->unbound_list);
>  		mutex_unlock(&group->unbound_lock);
> @@ -919,7 +916,7 @@ void *vfio_del_group_dev(struct device *dev)
>  	rc = try_wait_for_completion(&device->comp);
>  	while (rc <= 0) {
>  		if (device->ops->request)
> -			device->ops->request(device_data, i++);
> +			device->ops->request(device->device_data, i++);
> 
>  		if (interrupted) {
>  			rc = wait_for_completion_timeout(&device->comp,
> @@ -929,7 +926,7 @@ void *vfio_del_group_dev(struct device *dev)
>  				&device->comp, HZ * 10);
>  			if (rc < 0) {
>  				interrupted = true;
> -				dev_warn(dev,
> +				dev_warn(device->dev,
>  					 "Device is currently in use, task"
>  					 " \"%s\" (%d) "
>  					 "blocked until device is released",
> @@ -962,9 +959,17 @@ void *vfio_del_group_dev(struct device *dev)
> 
>  	/* Matches the get in vfio_group_create_device() */
>  	vfio_group_put(group);
> +}
> +EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
> +
> +void *vfio_del_group_dev(struct device *dev)
> +{
> +	struct vfio_device *device = dev_get_drvdata(dev);
> +	void *device_data = device->device_data;
> +
> +	vfio_unregister_group_dev(device);
>  	dev_set_drvdata(dev, NULL);

Move to vfio_unregister_group_dev? In the cover letter you mentioned
that drvdata is managed by the driver but removed from the core. Looks
it's also the rule obeyed by the following patches.

Thanks
Kevin

>  	kfree(device);
> -
>  	return device_data;
>  }
>  EXPORT_SYMBOL_GPL(vfio_del_group_dev);
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index b7e18bde5aa8b3..ad8b579d67d34a 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -15,6 +15,18 @@
>  #include <linux/poll.h>
>  #include <uapi/linux/vfio.h>
> 
> +struct vfio_device {
> +	struct device *dev;
> +	const struct vfio_device_ops *ops;
> +	struct vfio_group *group;
> +
> +	/* Members below here are private, not for driver use */
> +	refcount_t refcount;
> +	struct completion comp;
> +	struct list_head group_next;
> +	void *device_data;
> +};
> +
>  /**
>   * struct vfio_device_ops - VFIO bus driver device callbacks
>   *
> @@ -48,11 +60,15 @@ struct vfio_device_ops {
>  extern struct iommu_group *vfio_iommu_group_get(struct device *dev);
>  extern void vfio_iommu_group_put(struct iommu_group *group, struct
> device *dev);
> 
> +void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
> +			 const struct vfio_device_ops *ops, void
> *device_data);
> +int vfio_register_group_dev(struct vfio_device *device);
>  extern int vfio_add_group_dev(struct device *dev,
>  			      const struct vfio_device_ops *ops,
>  			      void *device_data);
> 
>  extern void *vfio_del_group_dev(struct device *dev);
> +void vfio_unregister_group_dev(struct vfio_device *device);
>  extern struct vfio_device *vfio_device_get_from_dev(struct device *dev);
>  extern void vfio_device_put(struct vfio_device *device);
>  extern void *vfio_device_data(struct vfio_device *device);
> --
> 2.30.2