Created Building for OpenBSD (markdown)

Building-for-OpenBSD.md

@@ -0,0 +1,43 @@
+*OpenBSD is not officially supported. These instructions are not actively updated and represent what happened to work for one user on amd64 in March 2018.*
+
+*Games do not currently work on OpenBSD/amd64 due to [bug 10926][1]. Until/unless a better solution is devised, users can apply a patch like the following to get past the bug:*
+```
+diff --git a/Source/Core/Common/MemArena.cpp b/Source/Core/Common/MemArena.cpp
+index 750a240e19..da77328e03 100644
+--- a/Source/Core/Common/MemArena.cpp
++++ b/Source/Core/Common/MemArena.cpp
+@@ -123,7 +123,7 @@ u8* MemArena::FindMemoryBase()
+ #if _ARCH_32
+   const size_t memory_size = 0x31000000;
+ #else
+-  const size_t memory_size = 0x400000000;
++  const size_t memory_size = 0x100000000;
+ #endif
+ 
+ #ifdef _WIN32
+```
+[1]: https://bugs.dolphin-emu.org/issues/10926
+
+Install the required packages: (Please amend if some are missing, I did not test this on a clean installation!)
+```
+pkg_add cmake pulseaudio qtbase
+```
+To build, run the following in the repository root directory:
+```
+mkdir build && cd build
+export Qt5Gui_DIR=/usr/local/lib/qt5/cmake/Qt5Gui
+export Qt5Widgets_DIR=/usr/local/lib/qt5/cmake/Qt5Widgets
+cmake \
+    -DCMAKE_EXE_LINKER_FLAGS='-L/usr/local/lib -lintl -z wxneeded' \
+    -DENABLE_WX=OFF ..
+make
+```
+Rationale:
++ Setting `Qt5Gui_DIR` and `Qt5Widgets_DIR` is necessary because CMake does not know to look for them under `/usr/local/lib/qt5/cmake` by default.
++ PulseAudio wants to link with libintl, but it is not included in the default configuration.
++ Linking with `-z wxneeded` is needed to enable writable-executable memory on OpenBSD. The resulting binary still needs to be run a partition with `wxallowed` set in the mount flags; `/usr/local` is mounted like this by default.
++ wxWidgets needs to be disabled because wxWidgets3 has not been ported to OpenBSD yet. The experimental Qt interface is the only working GUI.
+
+Other considerations:
++ Dolphin needs a lot of memory to run - more than OpenBSD's default limits allow. Increase the `datasize` limits for your login class to at least 4G in `/etc/login.conf`, or (assuming your login class's `datasize-max` limit permits this) run `ulimit -d 4194304` before running Dolphin, e.g. in your `~/.profile` file or in a Dolphin startup script.
++ To improve performance, you may want to add `-DCMAKE_CXX_FLAGS=-Ofast` to the above CMake invocation when building.