Re: RFC: WMI Enhancements

[Darren Hart <dvhart@infradead.org>]

On Thu, Apr 13, 2017 at 03:55:01PM +0000, Mario.Limonciello@dell.com wrote:
> 
> 
> > -----Original Message-----
> > From: Andy Lutomirski [mailto:luto@kernel.org]
> > Sent: Thursday, April 13, 2017 10:33 AM
> > To: Michał Kępień <kernel@kempniu.pl>
> > Cc: Darren Hart <dvhart@infradead.org>; Rafael Wysocki <rjw@rjwysocki.net>;
> > Len Brown <len.brown@intel.com>; Pali Rohár <pali.rohar@gmail.com>; Corentin
> > Chary <corentin.chary@gmail.com>; Limonciello, Mario
> > <Mario_Limonciello@Dell.com>; Andy Lutomirski <luto@kernel.org>; Andy
> > Shevchenko <andriy.shevchenko@linux.intel.com>; LKML <linux-
> > kernel@vger.kernel.org>; platform-driver-x86@vger.kernel.org; linux-
> > pm@vger.kernel.org
> > Subject: Re: RFC: WMI Enhancements
> > 
> > On Thu, Apr 13, 2017 at 12:32 AM, Michał Kępień <kernel@kempniu.pl> wrote:
> > >> Hi All,
> > >>
> > >> There are a few parallel efforts involving the Windows Management
> > >> Instrumentation (WMI)[1] and dependent/related drivers. I'd like to
> > >> have a round of discussion among those of you that have been involved
> > >> in this space before we decide on a direction.
> > >>
> > >> The WMI support in the kernel today fairly narrowly supports a
> > >> handful of systems. Andy L. has a work-in-progress series [2] which
> > >> converts wmi into a platform device and a proper bus, providing
> > >> devices for dependent drivers to bind to, and a mechanism for sibling devices to
> > communicate with each other.
> > >> I've reviewed the series and feel like the approach is sound, I plan
> > >> to carry this series forward and merge it (with Andy L's permission).
> > >>
> > >> Are there any objections to this?
> > >
> > > Back in January 2016, I sent Andy a few minor comments about this
> > > series.  A year later, I offered to iron out the remaining issues and
> > > resubmit the series in Andy's name when I find the time.  Sadly,
> > > things have changed a bit for me since that time and it is unlikely
> > > that I will be able to deliver, for which I am sorry.
> > >
> > > However, browsing Andy's branch I see that most issues have been
> > > resolved, though I think some of my remarks [1] have either been
> > > missed or silently refuted :)
> > >
> > > Anyway, I also like this approach and I think this series is a
> > > valuable cleanup.
> > 
> > Me too :)
> > 
> > >> In Windows, applications interact with WMI more or less directly. We
> > >> don't do this in Linux currently, although it has been discussed in
> > >> the past [3]. Some vendors will work around this by performing
> > >> SMI/SMM, which is inefficient at best. Exposing WMI methods to
> > >> userspace would bring parity to WMI for Linux and Windows.
> > >>
> > >> There are two principal concerns I'd appreciate your thoughts on:
> > >>
> > >> a) As an undiscoverable interface (you need to know the method
> > >> signatures ahead of time), universally exposing every WMI "device" to
> > >> userspace seems like "a bad idea" from a security and stability
> > >> perspective. While access would certainly be privileged, it seems
> > >> more prudent to make this exposure opt-in. We also handle some of
> > >> this with kernel drivers and exposing those "devices" to userspace
> > >> would enable userspace and the kernel to fight over control. So - if
> > >> we expose WMI devices to userspace, I believe this should be done on
> > >> a case by case basis, opting in, and not by default as part of the
> > >> WMI driver (although it can provide the mechanism for a sub-driver to use), and
> > possibly a devmode to do so by default.
> > 
> > I agree.  I don't want too see gnome-whatever-widget talking directly to WMI and
> > confusing the kernel driver for the same thing.
> 
> So there are plenty of other things that can be done by WMI that don't
> really make sense to live in the kernel, particularly on what Dell exposes via
> WMI.
> 
> If the desire of this group ends up being to not expose WMI by default,
> I'd like to at least propose it be exposed for the GUID's Dell is using.
> 

What I'm thinking is an explicit list of GUIDs within the drivers which are to
be exposed to user space. The rationale being:

* GUIDs which are managed by kernel drivers (LEDs, hotkeys, etc.) should not be
  exposed to userspace.

* Management GUIDs should not change frequently

* Management GUIDs are a trivial add, equivalent to adding a DEVICE ID to an
  existing driver. This means minimal review time to get upstream, and the
  ability to include in stable backports as needed. I haven't confirmed
  this with Greg KH, but I think I can make the case, especially after
  Andy L's WMI-as-a-bus patches.

> Perhaps as part of changing dell-smbios to use WMI, also extend it's
> functionality to userspace.

That would be consistent with the above in my opinion.

-- 
Darren Hart
VMware Open Source Technology Center