ksim

The little simulator that could.

ksim is a simulator for Intel Skylake GPUs. It grew out of two tools I wrote a while back for capturing and decoding the output of the Intel open source drivers. Once you're capturing and decoding the command stream output by the driver it's a small step to start interpreting the stream. Initially, it was exciting to see it fetch vertices, but it has now snowballed into a fairly competent and efficient software rasterizer. As of March 22nd, ksim runs all major GL/Vulkan shader stages (vertex, hull, domain, geometry and fragment shaders) as well as compute shaders. It JIT compiles the Intel GPU ISA to AVX2 code on-the-fly using its own IR and compiler.

ksim is far from complete and probably never will be. It was never intended to be useful but it's been, and continue to be, a fun and educational project. That said, many half-baked parts of ksim are falling into place and functionality I thought I'd never enable is now running. One day it might be useful as a way to provide a simulated GPU for Qemu or, together with the Intel open source Vulkan driver, a Vulkan software rasterizer.

Building and running

ksim uses the meson build system so after cloning the repo building it boils down to:

$ mkdir build
$ meson . build --buildtype=release
$ ninja -C build
$ sudo ninja -C build install

This sets up a meson release build, which is going build a much faster ksim, but for debugging you'll want --buildtype=debug. On Fedora, the ninja tool is called ninja-build.

Once installed, it's easy to launch any GL or Vulkan application under ksim:

$ ksim glxgears

will bring up the old gears running on ksim. Since ksim simulates the GPU from the kernel level down, it can also run Vulkan applications. Some of the demos [email protected] Willems vulkan demos run and look mostly right.

The ksim executable is a small shell script that sets up LD_PRELOAD so as to load the ksim shared object into the application and then launches the application. It follows the convention of similar launchers:

$ ksim [ksim args] [--] application [application args]

The ksim args may change on a whim, but typically you can pass --trace=FLAGS, with FLAGS being something like avx,gs,gem which makes ksim print a lot of info about the given topics.