Re: [PATCH v4] Drivers: hv: vmbus: Expose monitor data only when monitor pages are used

[Kimberly Brown <kimbrownkd@gmail.com>]

On Sun, Mar 03, 2019 at 09:05:43AM +0100, Greg KH wrote:
> On Fri, Mar 01, 2019 at 02:18:24PM -0500, Kimberly Brown wrote:
> > +/*
> > + * Channel-level attribute_group callback function. Returns the permission for
> > + * each attribute, and returns 0 if an attribute is not visible.
> > + */
> > +static umode_t vmbus_chan_attr_is_visible(struct kobject *kobj,
> > +					  struct attribute *attr, int idx)
> > +{
> > +	const struct vmbus_channel *channel =
> > +		container_of(kobj, struct vmbus_channel, kobj);
> > +
> > +	/* Hide the monitor attributes if the monitor mechanism is not used. */
> > +	if (!channel->offermsg.monitor_allocated &&
> > +	    (attr == &chan_attr_pending.attr ||
> > +	     attr == &chan_attr_latency.attr ||
> > +	     attr == &chan_attr_monitor_id.attr))
> > +		return 0;
> > +
> > +	return attr->mode;
> > +}
> > +
> > +static struct attribute_group vmbus_chan_group = {
> > +	.attrs = vmbus_chan_attrs,
> > +	.is_visible = vmbus_chan_attr_is_visible
> > +};
> > +
> >  static struct kobj_type vmbus_chan_ktype = {
> >  	.sysfs_ops = &vmbus_chan_sysfs_ops,
> >  	.release = vmbus_chan_release,
> > -	.default_attrs = vmbus_chan_attrs,
> 
> Why did you remove this line?
 
I removed the default attributes because vmbus_chan_attrs contains
non-default attributes. You suggested that I use one attribute_group and
an is_visible() callback for the device-level attributes (see
https://lore.kernel.org/lkml/20190226081848.GA15659@kroah.com/). I
assumed (possibly incorrectly) that I should do the same for these
channel-level attributes.


> >  };
> >  
> >  /*
> > @@ -1571,6 +1624,12 @@ int vmbus_add_channel_kobj(struct hv_device *dev, struct vmbus_channel *channel)
> >  	if (ret)
> >  		return ret;
> >  
> > +	ret = sysfs_create_group(kobj, &vmbus_chan_group);
> 
> Why are you adding these "by hand"?  What was wrong with using the
> default attribute group pointer?  You also are not removing the
> attributes :(

Are you referring to default_attrs in kobj_type? It's not an
attribute_group pointer, it's a pointer to an attribute pointer array.
The problem with using default_attrs is that all of the attributes are
visible.

I'm fairly certain that the monitor attributes are being removed.
sysfs_create_group() uses the attribute_group's is_visible() callback to
control the attribute visibility. And, when I look at the sysfs files, I
can see that the monitor sysyfs files are removed.

In v3, I proposed moving the monitor attributes to a special
attribute_group and adding that group manually when needed. Do you
prefer that approach for the channel-level monitor attributes?

Thanks,
Kim


> 
> greg k-h