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<chapter id="chap-X11-Application-Support">
  <title>X11 Application Support</title>
  <section id="sect-X11-Application-Support-introduction">
    <title>Introduction</title>
    <para>
      Being able to run existing X11 applications is crucial for the adoption
      of Wayland, especially on desktops, as there will always be X11
      applications that have not been or cannot be converted into Wayland
      applications, and throwing them all away would be prohibitive.
      Therefore a Wayland compositor often needs to support running X11
      applications.
    </para>
    <para>
      X11 and Wayland are different enough that there is no "simple" way to
      translate between them. Most of X11 is uninteresting to a Wayland
      compositor. That, combined with the gigantic implementation effort needed
      to support X11, makes it intractable to just write X11 support directly in
      a Wayland compositor. The implementation would be nothing short of a
      real X11 server.
    </para>
    <para>
      Therefore, Wayland compositors should use Xwayland, the X11 server that
      lives in the Xorg server source code repository and shares most of the
      implementation with the Xorg server. Xwayland is a complete X11 server,
      just like Xorg is, but instead of driving the displays and opening input
      devices, it acts as a Wayland client. The rest of this chapter talks
      about how Xwayland works.
    </para>
    <para>
      For integration and architecture reasons, while Xwayland is a Wayland
      client of the Wayland compositor, the Wayland compositor is an X11 client
      of Xwayland. This circular dependency requires special care from the
      Wayland compositor.
    </para>
  </section>
  <section id="sect-X11-Application-Support-two-modes">
    <title>Two Modes for Foreign Windows</title>
    <para>
      In general, windows from a foreign window system can be presented in one
      of two ways: rootless and rootful (not rootless).
    </para>
    <para>
      In rootful mode, the foreign window system as a whole is represented as a
      window (or more) of its own. You have a native window, inside which all
      the foreign windows are. The advantage of this approach in Xwayland's
      case is that you can run your favourite X11 window manager to manage your
      X11 applications. The disadvantage is that the foreign windows do not
      integrate with the native desktop. Therefore this mode is not usually
      used.
    </para>
    <para>
      In rootless mode, each foreign window is a first-class resident among the
      native windows. Foreign windows are not confined inside a native window
      but act as if they were native windows. The advantage is that one can
      freely stack and mix native and foreign windows, which is not possible in
      rootful mode. The disadvantage is that this mode is harder to implement
      and fundamental differences in window systems may prevent some things
      from working. With rootless Xwayland, the Wayland compositor must take
      the role as the X11 window manager, and one cannot use any other X11
      window manager in its place.
    </para>
    <para>
      This chapter concentrates on the rootless mode, and ignores the rootful
      mode.
    </para>
  </section>
  <section id="sect-X11-Application-Support-architecture">
    <title>Architecture</title>
    <para>
      A Wayland compositor usually takes care of launching Xwayland.
      Xwayland works in cooperation with a Wayland compositor as follows:
    </para>
    <figure>
      <title>Xwayland architecture diagram</title>
      <mediaobjectco>
	<imageobjectco>
	  <imageobject>
	    <imagedata fileref="images/xwayland-architecture.png" format="PNG" />
	  </imageobject>
	</imageobjectco>
      </mediaobjectco>
    </figure>
    <para>
      An X11 application connects to Xwayland just like it would connect to any
      X server. Xwayland processes all the X11 requests. On the other end,
      Xwayland is a Wayland client that connects to the Wayland compositor.
    </para>
    <para>
      The X11 window manager (XWM) is an integral part of the Wayland
      compositor. XWM uses the usual X11 window management protocol to manage
      all X11 windows in Xwayland. Most importantly, XWM acts as a bridge
      between Xwayland window state and the Wayland compositor's window manager
      (WWM). This way WWM can manage all windows, both native Wayland and X11
      (Xwayland) windows. This is very important for a coherent user
      experience.
    </para>
    <para>
      Since Xwayland uses Wayland for input and output, it does not have any
      use for the device drivers that Xorg uses. None of the xf86-video-* or
      xf86-input-* modules are used. There also is no configuration file for
      the Xwayland server. For optional hardware accelerated rendering,
      Xwayland uses GLAMOR.
    </para>
    <para>
      A Wayland compositor usually spawns only one Xwayland instance. This is
      because many X11 applications assume they can communicate with other X11
      applications through the X server, and this requires a shared X server
      instance. This also means that Xwayland does not protect nor isolate X11
      clients from each other, unless the Wayland compositor specifically
      chooses to break the X11 client intercommunications by spawning
      application specific Xwayland instances. X11 clients are naturally
      isolated from Wayland clients.
    </para>
    <para>
      Xwayland compatibility compared to a native X server will probably never
      reach 100%. Desktop environment (DE) components, specifically X11 window
      managers, are practically never supported. An X11 window manager would
      not know about native Wayland windows, so it could manage only X11
      windows. On the other hand, there must be an XWM that reserves the
      exclusive window manager role so that the Wayland compositor could show
      the X11 windows appropriately. For other DE components, like pagers and
      panels, adding the necessary interfaces to support them in WWM through XWM
      is often considered not worthwhile.
    </para>
  </section>
  <section id="sect-X11-Application-Support-xwm">
    <title>X Window Manager (XWM)</title>
    <para>
      From the X11 point of view, the X window manager (XWM) living inside a
      Wayland compositor is just like any other window manager. The difference
      is mostly in which process it resides in, and the few extra conventions
      in the X11 protocol to support Wayland window management (WWM)
      specifically.
    </para>
    <para>
      There are two separate asynchronous communication channels between
      Xwayland and a Wayland compositor: one uses the Wayland protocol, and the
      other one, solely for XWM, uses X11 protocol. This setting demands great
      care from the XWM implementation to avoid (random) deadlocks with
      Xwayland. It is often nearly impossible to prove that synchronous or
      blocking X11 calls from XWM cannot cause a deadlock, and therefore it is
      strongly recommended to make all X11 communications asynchronous. All
      Wayland communications are already asynchronous by design.
    </para>
    <section id="sect-X11-Application-Support-xwm-window-identification">
      <title>Window identification</title>
      <para>
	In Xwayland, an X11 window may have a corresponding wl_surface object
	in Wayland. The wl_surface object is used for input and output: it is
	referenced by input events and used to provide the X11 window content
	to the Wayland compositor. The X11 window and the wl_surface live in
	different protocol streams, and they need to be matched for XWM to do
	its job.
      </para>
      <para>
	When Xwayland creates a wl_surface on Wayland, it will also send an X11
	ClientMessage of type atom "WL_SURFACE_ID" to the X11 window carrying
	the wl_surface Wayland object ID as the first 32-bit data element. This
	is how XWM can associate a wl_surface with an X11 window. Note that
	the request to create a wl_surface and the ID message may arrive in any
	order in the Wayland compositor.
      </para>
    </section>
  </section>
</chapter>