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From:   Parav Pandit <parav@mellanox.com>
To:     Cornelia Huck <cohuck@redhat.com>
CC:     "alex.williamson@redhat.com" <alex.williamson@redhat.com>,
        Jiri Pirko <jiri@mellanox.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: RE: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
Thread-Topic: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
Thread-Index: AQHVXE6v3Mg2zMVvBUG6schRq+9Lx6cO0OsAgAADYqA=
Date:   Tue, 27 Aug 2019 11:07:37 +0000
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        <20190826204119.54386-4-parav@mellanox.com>
 <20190827124706.7e726794.cohuck@redhat.com>
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> -----Original Message-----
> From: Cornelia Huck <cohuck@redhat.com>
> Sent: Tuesday, August 27, 2019 4:17 PM
> To: Parav Pandit <parav@mellanox.com>
> Cc: alex.williamson@redhat.com; Jiri Pirko <jiri@mellanox.com>;
> kwankhede@nvidia.com; davem@davemloft.net; kvm@vger.kernel.org; linux-
> kernel@vger.kernel.org; netdev@vger.kernel.org
> Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
>=20
> On Mon, 26 Aug 2019 15:41:18 -0500
> Parav Pandit <parav@mellanox.com> wrote:
>=20
> > 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.
>=20
> What about
>=20
> "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 exa=
mple
> to create a netdevice name for the mdev."
>=20
Ok. I will change it.

> >
> > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > ---
> >  drivers/vfio/mdev/mdev_sysfs.c | 13 +++++++++++++
>=20
> I think the documentation should be updated as well.
>=20
> >  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 =3D mdev_from_dev(device);
> > +
> > +	if (!dev->alias)
> > +		return -EOPNOTSUPP;
>=20
> 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)?
No. This is confusing, to return empty value, because it says that there is=
 an alias but it is some weird empty string.
If there is alias, it shows exactly what it is.
If no alias it returns an error code =3D unsupported -> inline with other w=
idely used subsystem.

> - 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
New sysfs files can appear. Tool cannot say that I was not expecting this f=
ile here.
User space is supposed to work with the file they are off interest.
Mdev interface has option to specify vendor specific files, though in usual=
 manner it's not recommended.
So there is no old user space, new kernel issue here.

>   anyway.
>=20
> > +
> > +	return sprintf(buf, "%s\n", dev->alias); } static
> > +DEVICE_ATTR_RO(alias);
> > +
> >  static const struct attribute *mdev_device_attrs[] =3D {
> > +	&dev_attr_alias.attr,
> >  	&dev_attr_remove.attr,
> >  	NULL,
> >  };