RE: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree

[Parav Pandit]

> -----Original Message-----
> From: Cornelia Huck <cohuck@xxxxxxxxxx>
> Sent: Tuesday, August 27, 2019 5:05 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> 
> On Tue, 27 Aug 2019 11:07:37 +0000
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > > -----Original Message-----
> > > From: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > Sent: Tuesday, August 27, 2019 4:17 PM
> > > To: Parav Pandit <parav@xxxxxxxxxxxx>
> > > Cc: alex.williamson@xxxxxxxxxx; Jiri Pirko <jiri@xxxxxxxxxxxx>;
> > > kwankhede@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > linux- kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH 3/4] mdev: Expose mdev alias in sysfs tree
> > >
> > > On Mon, 26 Aug 2019 15:41:18 -0500
> > > Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > > > +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)?
> > 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 = unsupported -> inline with other widely
> 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 file
> 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.
> 
> I'm not talking about old userspace/new kernel, but new userspace/old kernel.
> Code that wants to consume this attribute needs to be able to cope with its
> absence anyway.
> 
Old kernel doesn't have alias file.
If some tool tries to read this file it will fail to open non existing file; open() system call is already taking care of it.