linux-kernel - Re: [RFC PATCH 1/7] vfio: Include optional device match in vfio_device

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Date:   Thu, 6 Feb 2020 12:14:19 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, dev@...k.org, mtosatti@...hat.com,
        thomas@...jalon.net, bluca@...ian.org, jerinjacobk@...il.com,
        bruce.richardson@...el.com
Subject: Re: [RFC PATCH 1/7] vfio: Include optional device match in
 vfio_device_ops callbacks

On Tue, 04 Feb 2020 16:05:43 -0700
Alex Williamson <alex.williamson@...hat.com> wrote:

> Allow bus drivers to provide their own callback to match a device to
> the user provided string.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  drivers/vfio/vfio.c  |   19 +++++++++++++++----
>  include/linux/vfio.h |    3 +++
>  2 files changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 388597930b64..dda1726adda8 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -875,11 +875,22 @@ EXPORT_SYMBOL_GPL(vfio_device_get_from_dev);
>  static struct vfio_device *vfio_device_get_from_name(struct vfio_group *group,
>  						     char *buf)
>  {
> -	struct vfio_device *it, *device = NULL;
> +	struct vfio_device *it, *device = ERR_PTR(-ENODEV);
>  
>  	mutex_lock(&group->device_lock);
>  	list_for_each_entry(it, &group->device_list, group_next) {
> -		if (!strcmp(dev_name(it->dev), buf)) {
> +		int ret;
> +
> +		if (it->ops->match) {
> +			ret = it->ops->match(it->device_data, buf);
> +			if (ret < 0 && ret != -ENODEV) {
> +				device = ERR_PTR(ret);
> +				break;
> +			}
> +		} else
> +			ret = strcmp(dev_name(it->dev), buf);

The asymmetric braces look a bit odd.

> +
> +		if (!ret) {
>  			device = it;
>  			vfio_device_get(device);
>  			break;
> @@ -1441,8 +1452,8 @@ static int vfio_group_get_device_fd(struct vfio_group *group, char *buf)
>  		return -EPERM;
>  
>  	device = vfio_device_get_from_name(group, buf);
> -	if (!device)
> -		return -ENODEV;
> +	if (IS_ERR(device))
> +		return PTR_ERR(device);
>  
>  	ret = device->ops->open(device->device_data);
>  	if (ret) {
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e42a711a2800..755e0f0e2900 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -26,6 +26,8 @@
>   *         operations documented below
>   * @mmap: Perform mmap(2) on a region of the device file descriptor
>   * @request: Request for the bus driver to release the device
> + * @match: Optional device name match callback (return: 0 for match, -ENODEV
> + *         (or >0) for no match and continue, other -errno: no match and stop)

I'm wondering about these conventions.

If you basically want a tri-state return (match, don't match/continue,
don't match/stop), putting -ENODEV and >0 together feels odd. I would
rather expect either
- < 0 == don't match/stop, 0 == don't match/continue, > 0 == match, or
- 0 == match, -ENODEV (or some other defined error) == don't
  match/continue, all other values == don't match/abort?

>   */
>  struct vfio_device_ops {
>  	char	*name;
> @@ -39,6 +41,7 @@ struct vfio_device_ops {
>  			 unsigned long arg);
>  	int	(*mmap)(void *device_data, struct vm_area_struct *vma);
>  	void	(*request)(void *device_data, unsigned int count);
> +	int	(*match)(void *device_data, char *buf);
>  };
>  
>  extern struct iommu_group *vfio_iommu_group_get(struct device *dev);
>