Re: [PATCH v1 0/2] libxl: add PV display device driver interface

[Ian Jackson <ian.jackson@eu.citrix.com>]

Oleksandr Grytsov writes ("Re: [Xen-devel] [PATCH v1 0/2] libxl: add PV display device driver interface"):
> On Thu, Apr 13, 2017 at 3:54 PM, Ian Jackson <ian.jackson@eu.citrix.com> wrote:
> > Can you sketch out what the rest of the system does, then ?
> > Presumably there has to be a different control protocol to your
> > backend, to tell it where to put the actual displays.  I think it
> > would be good if that protocol were the same for different use cases,
> > and documented somewhere.
> 
> Well, yes there will be some protocol to communicate with the window
> manager.  We consider to use wayland and weston as display system.
> The idea is the backend and other applications on the host render
> content onto wayland surfaces. Each surface will have a unique
> identifier or attribute (like multimedia, navigation etc.). The
> window manager based on these attributes, configured policies and
> scenarios will set positions and geometries for the surfaces.

That seems to make some kind of sense.  However, surely that means
that the backend needs to be told the "identifier or attribute" to
assign to the surfaces it is creating ?

Ie that would have to be in here somewhere
 vdispl = [ 'backend=0, devId=0, beAlloc=1, connectors=800x600,1024x768' ]

> > So for example, when a window manager moves a window, how is the
> > backend told where to display the guest output ?
> 
> The backend will create the wayland surface and tell the window manager
> the attribute it has. All rest will be done by the window manager.

Or to put my comment above another way: if there are several such
surfaces, how does the window manager tell which is intended for
what ?

> In my opinion the details how the virtual displays are placed on the host
> should be separated from xl configuration. Because it really depends on
> backend implementation. We have generalized display protocol (displif.h).
> There could be different frontend/backend implementations and each
> implementation will require its own configuration.
> That's why in xl.cfg should be only parameters related to display protocol
> namely resolutions of virtual connectors.

xl.cfg normally also contains information necessary for plumbing the
virtual device back to some physical hardware.

So for example, the disk specification specifies not only the names of
the virtual devices, but also the corresponding host physical devices
(or files), host data storage format, etc.

As I say I think in this case we probably want the configuration file
to assign names or ids to the backend surfaces.

Ian.

PS: Sorry for the slow reply.

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