libglvnd: the GL Vendor-Neutral Dispatch library

Introduction

This is a work-in-progress implementation of the vendor-neutral dispatch layer for arbitrating OpenGL API calls between multiple vendors on a per-screen basis, as described by Andy Ritger's OpenGL ABI proposal [1].

Currently, only the GLX window-system API and OpenGL are supported, but in the future this library may support EGL and OpenGL ES as well.

Building the library

Run ./autogen.sh. Run ./dbg_configure.sh to build the library in debug mode, or ./configure to build it in release mode. Finally, run make.

Code overview

The code in the src/ directory is organized as follows:

• GLX/ contains code for libGLX, the GLX window-system API library.
• GLdispatch/ contains code for libGLdispatch, which is really just a thin wrapper around Mesa's glapi that tries to hide most of the complexity of managing dispatch tables. Its interface is defined in GLdispatch.h. This implements the guts of the core GL API libraries.
• EGL/ and GLESv{1,2}/ are placeholders for now. GLESv{1,2}/ will be filter libraries on libGLdispatch, while EGL/ will contain libEGL, which may be implemented similarly to libGLX.
• GL/ and OpenGL/ respectively contain code to generate libGL and libOpenGL, which are both merely filter libraries for libGLX and libGLdispatch.
• util/ contains generic utility code, and arch/ contains architecture-specific defines.

There are a few good starting points for familiarizing oneself with the code:

• Look at the vendor-library to GLX ABI defined in libglxabi.h.
• Follow the flow of glXGetProcAddress() -> __glDispatchGetProcAddress() -> __glapi_get_proc_address() to see how the dispatch table is updated as new GL stubs are generated, and how GLX looks for vendor-library-implemented dispatchers for GLX extension functions.
• Follow the flow of glXMakeContextCurrent() -> __glDispatchMakeCurrent() -> _glapi_set_current() to see how the current dispatch table and state is updated by the API library.
• Look at libglxmapping.c:__glXLookupVendorBy{Name,Screen}() to see how vendor library names are queried. At the same time, look at x11glvnd{client,server}.c to see how the "GLVendor" extension which retrieves the appropriate mappings is implemented.

The tests/ directory contains several unit tests which verify that dispatching to different vendors actually works. Run make check to run these unit tests. Note some of the unit tests require a special environment that can only be set up if X is not currently running. To skip these tests (and environment initialization/teardown), run make check SKIP_ENV_INIT=1.

Architecture

The library organization differs slightly from that of Andy's original proposal. See the diagram below:

                ┌──────────────────────────────────┐
                │                                  │
          ┌─────┤        Application               │
          │     │                                  │
          │     └─────┬───────────────────┬────────┘
          │           │                   │
          │     ┌─────▾─────┐             │                    ┌──────────────┐
          │     │           │             │                    │              │
          │     │ libOpenGL │             │                    │              │
          │     │           │             │                    │  X server    │
          │     └─────┬─────┘             │                    │              │
          │        DT_FILTER              │                    │              │
          │     ┌─────▾──────────┐ ┌──────▾────────┐           │ ┌──────────┐ │
          │     │                │ │               │           └─│GLVendor  │─┘
          │     │ [mapi/glapi]   ◂─▸               │             │extension │
          │     │ libGLdispatch  │ │   libGLX      ├─────────────▸──────────┘
          │     │                │ │               ◂──────────┬─────────────────┐
          │     └───────▴────────┘ └──────▴────────┘          │                 │
          │         DT_FILTER         DT_FILTER             ┌─▾─────────┐   ┌───▾────────┐
          │     ┌───────┴─────────────────┴────────┐        │           │   │            │
          │     │                                  │        │           │   │            │
          └─────▸           libGL                  │        │ GLX_vendor│   │ GLX_vendor2│
                └──────────────────────────────────┘        │           │   │            │
                                                            └───────────┘   └────────────┘

In this diagram,

• A ───▸ B indicates that module A calls into module B.
• A ── DT_FILTER ──▸ B indicates that DSO A is a filter library on DSO B, and symbols exported by A are resolved via ELF symbol filtering to entrypoints in B.

libGLX manages loading GLX vendor libraries and dispatching GLX core and extension functions to the right vendor.

GLVendor is a simple X extension which allows libGLX to determine the number of the screen belonging to an arbitrary drawable XID, and also the GL vendor to use for a given screen.

libGLdispatch implements core GL dispatching and TLS. It acts as a thin wrapper around glapi which provides some higher-level functionality for managing dispatch tables, requesting vendor proc addresses, and making current to a given context + dispatch table. This is a separate library rather than statically linked into libGLX, since current dispatch tables will eventually be shared between GLX and EGL, similarly to how glapi operates when Mesa is compiled with the --shared-glapi option.

libOpenGL is a filter library on libGLdispatch which exposes OpenGL 4.x core and compatibility entry points. Eventually, there will be a libGLESv{1,2} which will also be filter libraries on libGLdispatch that expose GLES entry points.

libGL is a filter library on libGLdispatch and libGLX which is provided for backwards-compatibility with applications which link against the old ABI.

NOTE: Logically, libGL should be a filter library on libOpenGL rather than libGLdispatch, as libGLdispatch is an implementation detail. The current arrangement is in place to work around a loader bug (present on at least ld.so v.2.12) where symbols in libGL don't properly resolve to libGLdispatch symbols if it filters libOpenGL instead of libGLdispatch.

GLX dispatching

Unlike core OpenGL functions, whose vendor can be determined from the current context, many GLX functions are context-independent. In order to successfully map GLX API calls to the right vendor, we use the following strategy:

• Most GLX entry points specify (either explicitly, or implicitly) an X screen.

• On a per-entry point basis, dispatch the call to the libGLX_VENDOR.so for that screen.

• The first time libGLX.so gets called with a unique combination of X Display + screen, do the following:

  • Use the Display connection to query the X server for the GLX vendor of that X screen.

  • Load the correspending libGLX_VENDOR.so.

  • Read the vendor's GLX dispatch table from the libGLX_VENDOR.so.

  • Cache that Display + screen <=> vendor dispatch table mapping, for use in subsequent dispatching.

• Some GLX entry points imply an X screen by a GLX object they specify. Such GLX objects are:

  GLXContext (an opaque pointer) GLXFBConfig (an opaque pointer) GLXPixmap (an XID) GLXDrawable (an XID) GLXWindow (an XID) GLXPbuffer (an XID)

  To map from object to screen, record the corresponding screen when the object is created. This means the current process needs to see a GLX call to create the object. In the case of the opaque pointers, this is reasonable, since the pointer is only meaningful within the current process. But XIDs could be created by another process. See the Issues section below.

• To minimize code complexity from error checking, define a noop GLX dispatch table. This is returned by __glXGet{,Current}Dispatch() in case no other dispatch table can be found.

• Similarly, __glXScreenFrom{Context,FBConfig,Drawable}() may fail to find a screen matching the specified GLX object. In this case, the returned screen number is -1, but the caller should just pass the screen number through to __glXGetDispatch() or __glX{Add,Remove}Screen{Context,FBConfig,Drawable}Mapping(). Those functions are expected to deal gracefully with the invalid screen number.

Issues

• The library currently indirectly associates a drawable with a vendor, by first mapping a drawable to its screen, then mapping the screen to its vendor. However, it may make sense in render offload scenarios to allow direct mapping from drawables to vendors, so multiple vendors could potentially operate on drawables in the same screen. The problem with this is that several GLX functions, such as glXChooseFBConfig(), explicitly refer to screens, and so it becomes a gray area which vendor the call should be dispatched to. Given this issue, does it still make more sense to use a direct drawable to vendor mapping? How would this be implemented? Should we add new API calls to "GLX Next"?

• Along the same lines, would it be useful to include a "glXGetProcAddressFromVendor()" or "glXGetProcAddressFromScreen()" entrypoint in a new GLX version to obviate the need for this library in future applications?

• Global state is required by both libGLX.so and libGLdispatch.so for various purposes, and needs to be protected by locks in multithreaded environments. Is it reasonable for the vendor-neutral library to depend on pthreads for implementing these locks?

  While there is no harm in having the API libraries link against pthreads even if the application does not, we would like to avoid pthread locking overhead if the application is single-threaded. Hence, this library uses a glvnd_pthread wrapper library which provides single-threaded fallbacks for applications which are not linked against pthreads. It is expected that multi-threaded applications will either statically link against pthreads, or load pthreads prior to loading libGL.

• Is using a hash table to store GLX extension entrypoints performant enough for dispatching? Should we be using a flat array instead?

• How should malloc(3) failures be handled?

• How should forking be handled?

• Should we map XIDs directly to vendors, rather than to screens?

• The current libGLX implementation stores the mapping between screen and all objects of the same type in one hash table. I.e., all pointer types (GLXContext and GLXFBConfig) in one table, and all XID types (GLXDrawable, GLXPixmap, GLXWindow, and GLXPbuffer) in another table. Should there instead be more finer-grained hash tables? There probably couldn't be finer-grained tables for XIDs, because GLXDrawable is used interchangably with the other XID-based types.

• The issue above applies to XIDs in the x11glvnd extension as well: we currently don't make any distinction between window, GLX pixmap, GLX window, or GLX pbuffer XIDs.

• Querying an XID <=> screen mapping without somehow "locking" the XID is inherently racy, since a different process may destroy the drawable, and X may recycle the XID, after the mapping is saved client-side. Is there a mechanism we could use to notify the API library when a mapping is no longer valid?

• Currently the library does not attempt to clean up allocations and unload vendor libraries if the application unloads it. This will need to be implemented eventually for the library to be usable in a production environment. What will the sequencing of this look like? Should we also hook into XCloseDisplay()?

• Should x11glvnd be an extension on top of GLX 1.4, or a "GLX Next" feature?

TODO

• Refactor so the core OpenGL dispatch table, and the vendor-library ABI exposed to manipulate this table, is independent of the libGLX ABI.

• Fix glXGetClientString() to take a real "union" of strings, rather than just concatenating them.

• Implement libEGL, and libGLESv{1,2}.

• Root-cause and fix the loader bug that prevents libGL from filtering libOpenGL rather than libGLdispatch to pick up core GL symbols.

• Currently, running "ldd libGL.so" fails with the following assertion (for ldd v.2.15 on Ubuntu 12.04 LTS):

  Inconsistency detected by ld.so: dl-deps.c: 592: _dl_map_object_deps: Assertion `map->l_searchlist.r_list[0] == map' failed!

  Root-cause and fix this bug.

References

[1] https://github.com/aritger/linux-opengl-abi-proposal/blob/master/linux-opengl-abi-proposal.txt

Acknowledgements

Thanks to Andy Ritger for the original libGLX implementation and README documentation.

X.Org

libglvnd contains list.h, a linked list implementation from the X.Org project. Source code from the X.Org project is available from:

http://cgit.freedesktop.org/xorg/xserver

list.h carries the following license:

Copyright © 2010 Intel Corporation
Copyright © 2010 Francisco Jerez <[email protected]>

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice (including the next
paragraph) shall be included in all copies or substantial portions of the
Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
IN THE SOFTWARE.

Mesa

libglvnd contains code from the Mesa project. Source code from the Mesa project is available from:

http://cgit.freedesktop.org/mesa/mesa

The default Mesa license is as follows:

Copyright (C) 1999-2007  Brian Paul   All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
BRIAN PAUL BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

uthash

libglvnd uses the hash table implementation 'uthash':

http://troydhanson.github.io/uthash/

This library carries the following copyright notice:

Copyright (c) 2005-2013, Troy D. Hanson
http://troydhanson.github.com/uthash/
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

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NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

buildconf

libglvnd uses the buildconf autotools bootstrapping script 'autogen.sh':

http://freecode.com/projects/buildconf

This script carries the following copyright notice:

Copyright (c) 2005-2009 United States Government as represented by
the U.S. Army Research Laboratory.

Redistribution and use in source and binary forms, with or without
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3. The name of the author may not be used to endorse or promote
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ax-pthread

libglvnd uses the AX_PTHREAD autoconf macro for detecting pthreads. The implementation of this macro carries the following license:

Copyright (c) 2008 Steven G. Johnson <[email protected]>
Copyright (c) 2011 Daniel Richard G. <[email protected]>

This program is free software: you can redistribute it and/or modify it
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Free Software Foundation, either version 3 of the License, or (at your
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This program is distributed in the hope that it will be useful, but 
WITHOUT ANY WARRANTY; without even the implied warranty of
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You should have received a copy of the GNU General Public License along
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