Our UXA benchmarks have shown that this interface is significantly faster than EXA, but it's not yet free of bugs. Those using UXA are frequently prone to graphical corruption, X Server crashes, and other problems. The UMA Acceleration Architecture has been around for a number of months now, but the situation isn't better yet.
Bryce Harrington, the lead X.Org engineer at Canonical, has decided these performance improvements do not outweigh Intel's UXA bugs and as a result it will not be enabled by default in Ubuntu 9.04. Bryce shares that some Ubuntu users are able to use UXA without any problems, while others with the same Intel IGPs report serious regressions. He hopes, however, to enable UXA by default on Intel hardware in Ubuntu 9.10. Those not wishing to use EXA on Intel hardware still can change the acceleration type to UXA from their xorg.conf.
More on the Ubuntu decisions about Intel UXA can be found on the Ubuntu-X mailing list and on the UxaTesting Wiki. Originally UXA was supposed to be merged back into EXA once the Intel open-source engineers decided how to deal with kernel memory management and 2D acceleration, but we found out earlier this year that Intel doesn't intend to eliminate UXA.