Subject: Re: [PATCH V9 mlx5-next 08/15] vfio: Have the core code decode the VFIO_DEVICE_FEATURE ioctl

From: Cornelia Huck <[email-masked]>
Date: Wed, 02 Mar 2022 11:00:08 +0100

On Thu, Feb 24 2022, Yishai Hadas <[email-masked]> wrote:

> From: Jason Gunthorpe <[email-masked]>
>
> Invoke a new device op 'device_feature' to handle just the data array
> portion of the command. This lifts the ioctl validation to the core code
> and makes it simpler for either the core code, or layered drivers, to
> implement their own feature values.
>
> Provide vfio_check_feature() to consolidate checking the flags/etc against
> what the driver supports.
>
> Signed-off-by: Jason Gunthorpe <[email-masked]>
> Tested-by: Shameer Kolothum <[email-masked]>
> Signed-off-by: Yishai Hadas <[email-masked]>
> ---
>  drivers/vfio/pci/vfio_pci.c      |  1 +
>  drivers/vfio/pci/vfio_pci_core.c | 94 +++++++++++++-------------------
>  drivers/vfio/vfio.c              | 46 ++++++++++++++--
>  include/linux/vfio.h             | 32 +++++++++++
>  include/linux/vfio_pci_core.h    |  2 +
>  5 files changed, 114 insertions(+), 61 deletions(-)

(...)

> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 76191d7abed1..ca69516f869d 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -55,6 +55,7 @@ struct vfio_device {
>   * @match: Optional device name match callback (return: 0 for no-match, >0 for
>   *         match, -errno for abort (ex. match with insufficient or incorrect
>   *         additional args)
> + * @device_feature: Fill in the VFIO_DEVICE_FEATURE ioctl
>   */
>  struct vfio_device_ops {
>  	char	*name;
> @@ -69,8 +70,39 @@ struct vfio_device_ops {
>  	int	(*mmap)(struct vfio_device *vdev, struct vm_area_struct *vma);
>  	void	(*request)(struct vfio_device *vdev, unsigned int count);
>  	int	(*match)(struct vfio_device *vdev, char *buf);
> +	int	(*device_feature)(struct vfio_device *device, u32 flags,
> +				  void __user *arg, size_t argsz);
>  };

Is the expectation that most drivers will eventually implement
->device_feature()? Well, they will have to if they want to support
migration; mostly asking because e.g. ->match() is explicitly marked as
"optional". As the only callback every driver implements seems to be
->ioctl() (if we also include the samples), "optional" or not does not
seem to be particularly relevant anyway.

Reviewed-by: Cornelia Huck <[email-masked]>