Re: [PATCH v3 1/7] vfio: allow external user to get vfio group from device

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 24 Feb 2020 03:46:41 -0500
Yan Zhao <yan.y.zhao@intel.com> wrote:

> external user is able to
> 1. add a device into an vfio group

How so?  The device is added via existing mechanisms, the only thing
added here is an interface to get a group reference from a struct
device.

> 2. call vfio_group_get_external_user_from_dev() with the device pointer
> to get vfio_group associated with this device and increments the container
> user counter to prevent the VFIO group from disposal before KVM exits.
> 3. When the external KVM finishes, it calls vfio_group_put_external_user()
> to release the VFIO group.
> 
> Suggested-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  drivers/vfio/vfio.c  | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/vfio.h |  2 ++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index c8482624ca34..914bdf4b9d73 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1720,6 +1720,43 @@ struct vfio_group *vfio_group_get_external_user(struct file *filep)
>  }
>  EXPORT_SYMBOL_GPL(vfio_group_get_external_user);
>  
> +/**
> + * External user API, exported by symbols to be linked dynamically.
> + *
> + * The protocol includes:
> + * 1. External user add a device into a vfio group
> + *
> + * 2. The external user calls vfio_group_get_external_user_from_dev()
> + * with the device pointer
> + * to verify that:
> + *	- there's a vfio group associated with it and is initialized;
> + *	- IOMMU is set for the vfio group.
> + * If both checks passed, vfio_group_get_external_user_from_dev()
> + * increments the container user counter to prevent
> + * the VFIO group from disposal before KVM exits.
> + *
> + * 3. When the external KVM finishes, it calls
> + * vfio_group_put_external_user() to release the VFIO group.
> + * This call decrements the container user counter.
> + */

I don't think we need to duplicate this whole comment block for a
_from_dev() version of the existing vfio_group_get_external_user().
Please merge the comments.

> +
> +struct vfio_group *vfio_group_get_external_user_from_dev(struct device *dev)
> +{
> +	struct vfio_group *group;
> +	int ret;
> +
> +	group = vfio_group_get_from_dev(dev);
> +	if (!group)
> +		return ERR_PTR(-ENODEV);
> +
> +	ret = vfio_group_add_container_user(group);
> +	if (ret)
> +		return ERR_PTR(ret);

Error path leaks group reference.

> +
> +	return group;
> +}
> +EXPORT_SYMBOL_GPL(vfio_group_get_external_user_from_dev);
> +
>  void vfio_group_put_external_user(struct vfio_group *group)
>  {
>  	vfio_group_try_dissolve_container(group);
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e42a711a2800..2e1fa0c7396f 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -94,6 +94,8 @@ extern void vfio_unregister_iommu_driver(
>   */
>  extern struct vfio_group *vfio_group_get_external_user(struct file *filep);
>  extern void vfio_group_put_external_user(struct vfio_group *group);
> +extern
> +struct vfio_group *vfio_group_get_external_user_from_dev(struct device *dev);

Slight cringe at this line wrap, personally would prefer to wrap the
args as done repeatedly elsewhere in this file.  Thanks,

Alex

>  extern bool vfio_external_group_match_file(struct vfio_group *group,
>  					   struct file *filep);
>  extern int vfio_external_user_iommu_id(struct vfio_group *group);