netdev - Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree

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Date:   Tue, 27 Aug 2019 12:47:06 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Parav Pandit <parav@...lanox.com>
Cc:     alex.williamson@...hat.com, jiri@...lanox.com,
        kwankhede@...dia.com, davem@...emloft.net, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree

On Mon, 26 Aug 2019 15:41:18 -0500
Parav Pandit <parav@...lanox.com> wrote:

> Expose mdev alias as string in a sysfs tree so that such attribute can
> be used to generate netdevice name by systemd/udev or can be used to
> match other kernel objects based on the alias of the mdev.

What about

"Expose the optional alias for an mdev device as a sysfs attribute.
This way, userspace tools such as udev may make use of the alias, for
example to create a netdevice name for the mdev."

> 
> Signed-off-by: Parav Pandit <parav@...lanox.com>
> ---
>  drivers/vfio/mdev/mdev_sysfs.c | 13 +++++++++++++

I think the documentation should be updated as well.

>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/vfio/mdev/mdev_sysfs.c b/drivers/vfio/mdev/mdev_sysfs.c
> index 43afe0e80b76..59f4e3cc5233 100644
> --- a/drivers/vfio/mdev/mdev_sysfs.c
> +++ b/drivers/vfio/mdev/mdev_sysfs.c
> @@ -246,7 +246,20 @@ static ssize_t remove_store(struct device *dev, struct device_attribute *attr,
>  
>  static DEVICE_ATTR_WO(remove);
>  
> +static ssize_t alias_show(struct device *device,
> +			  struct device_attribute *attr, char *buf)
> +{
> +	struct mdev_device *dev = mdev_from_dev(device);
> +
> +	if (!dev->alias)
> +		return -EOPNOTSUPP;

I'm wondering how to make this consumable by userspace in the easiest way.
- As you do now (userspace gets an error when trying to read)?
- Returning an empty value (nothing to see here, move along)?
- Or not creating the attribute at all? That would match what userspace
  sees on older kernels, so it needs to be able to deal with that
  anyway.

> +
> +	return sprintf(buf, "%s\n", dev->alias);
> +}
> +static DEVICE_ATTR_RO(alias);
> +
>  static const struct attribute *mdev_device_attrs[] = {
> +	&dev_attr_alias.attr,
>  	&dev_attr_remove.attr,
>  	NULL,
>  };