Jerome Glisse, who has long been involved with the development of open-source ATI X.Org drivers and was the original developer behind the short-lived xf86-video-avivo driver, has submitted a command stream state checker for the Radeon DRM code. With the ATI GPU memory management being done within the kernel, there must be safeguards in place that prevent user-space processes from exploiting that by accessing system memory beyond its allowed GPU memory objects.
This command stream state checker currently supports the ATI R300, R400, and R500 generations of hardware. This code checks against color buffers, z-buffers, vertex buffers, and textures. Right now this newly-proposed code does not guard against 2D rendering states. With the additional overhead now of having to monitor the GPU's command stream to ensure that no commands are violating their memory rights, there is a performance cost. Jerome mentions that he is currently seeing about a 3% decrease in performance, but further optimization can still be done via caching support.
Jerome's R300-R500 series command state checker is just over 1,000 lines of code long and can be found on the DRI development list.