The SNA support has been a long time coming going back two summers with Sandy Bridge. SNA is much faster than UXA, the default 2D acceleration method up to this point since it took over EXA when Intel migrated to GEM+KMS. SNA is much larger in size than UXA but the benchmarks across all generations of hardware we have tested have been really beneficial. Ubuntu and other distributions have already used SNA by default for performance reasons. Check out my many SNA benchmarks for more information on this Intel 2D acceleration method.
The change to use Intel SNA by default happened this morning with this Git commit.
For those having issues with SNA, UXA can still be enabled by setting the appropriate value in the AccelMethod of the xorg.conf.
The other big change is the merging of XMir handling in the xf86-video-intel driver. When using XMir for running X11/X.Org applications atop a Mir display server, modified DDX drivers are still required. These modifications are now present in the xf86-video-intel driver by default rather than Canonical carrying the work as out-of-tree patches.
This merging happened since Christopher James Halse Rogers has updated the XMir API and now claims that the XMir API should be stable unless there's "people coming up with more awesome ways of doing things."
This is the first X.Org DDX driver to merge XMir support and there still isn't any Mir platform support added to mainline Mesa. The Intel XMir support happened with this Git commit.
After making those two big changes this morning, Chris Wilson of Intel's Open-Source Technology Center then tagged 2.99.901. This is the first development snapshot of the xf86-video-intel 3.0 graphics driver. The SNA and XMir support are causing the major version number to be bumped.