The latest Intel Windows driver for Haswell goes from supporting OpenGL 4.1 to OpenGL 4.2 while right now with the current Mesa stable release there is OpenGL 3.1 compliance. Only with the Mesa 10.0 release due out in stable, released form in a few days will support for OpenGL 3.2 and OpenGL 3.3 be added to core Mesa and the i965 driver.
We will be lucky by this time next year if Intel's Linux driver has full OpenGL 4.1/4.2 support. The difference, however, with Intel's Linux driver over the closed-source NVIDIA (and AMD) drivers where the performance and OpenGL support can be the same between platforms is that the Intel driver code between Windows and Linux is not shared. Intel has their Windows team churning out (and generally leading) when it comes to OpenGL driver development for their integrated graphics hardware while the Intel Open-Source Technology Center has their own team working on the Linux driver that revolves around Mesa, DRM, X.Org, etc. Regardless, for end-users it would be nice if their drivers were closer to parity between platforms.
The good news is that while the Intel Linux driver is lagging behind the Windows driver, Intel's Linux developers remain dedicated towards the cause and in the days of Steam on Linux they have been working with Valve and acting more quickly when it comes to resolving Linux game bugs, performance issues, and OpenGL extensions needed by common game engines. Full OpenGL 4 support may be a ways off, but various OpenGL 4.1/4.2/4.3 extensions have already been committed to Mesa.
The latest GL4 extension to be proposed for core Mesa and the Intel driver is ARB_shader_image_load_store. ARB_shader_image_load_store is useful and allows for developers to load/store and perform atomic read-modify-write operations to a single level texture from an image from any shader stage. The OpenGL.org registry specification has more details.
The core Mesa support for ARB_shader_image_load_store was proposed on Sunday by Francisco Jerez with this mailing list patch-set. "This is the first of three patch series enabling basic support for ARB_shader_image_load_store on Intel Gen7 (and Gen7.5) hardware. Most of the necessary core mesa changes are part of this batch (except the one patch that depends on GLSL changes). I'll be sending a second batch with the GLSL compiler changes shortly, and in at most a few days a third, considerably more complex batch with the i965 driver and compiler back-end changes."
While on the topic of Intel Linux OpenGL support improvements, committed today was indirect drawing support for the latest Intel chips.
Besides needing to catch up to the Windows driver with OpenGL support, the Linux performance is a mixed bag when compared to Windows 8. The Intel Windows driver has just added other new features too like Intel Wireless Display Miracast support for wireless streaming and other optimizations.
At least the open-source Intel Linux driver is ahead of the Radeon and Nouveau Gallium3D drivers with their GL support that trail the chip giant (albeit the NVIDIA and AMD Catalyst drivers still offer the best performance and OpenGL support on Linux systems), but hopefully we'll see a nice push forward in 2014. For those wanting more details on the latest Intel Windows driver improvements, visit Geeks3D.com.