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In 2022 DirectX 12 support is a top priority. That being said, our CrossOver developers are working through the challenge of developing support for DirectX 12 in two distinct gaming environments. The obstacles in Linux are not the same as the obstacles in Mac. Let's take a look at how CrossOver supports DirectX 12 and what the challenges are.

CrossOver uses VKD3D to run DirectX 12 games. VKD3D is a 3D graphics library built on top of Vulkan. Currently, lots of work is being done to improve VKD3D performance. With the help of the Vulkan descriptor indexing extension, which allows for functionality similar to DirectX 12 descriptor heaps, Vulkan descriptors are written less often and far less GPU memory is used. As a result, VKD3D can support games that use enough descriptors to require resources from Tier 2 and Tier 3 hardware.

All that means support for DirectX 12 on Linux is arriving in 2022 with CrossOver 22. Support for DirectX 12 on Mac is another story. Why, you ask?

On Mac, there is an additional layer of complexity when trying to use VKD3D for DirectX 12 games. That layer of complexity is Metal. Metal is Apple’s low-level, low-overhead, hardware-accelerated graphic and compute shader API. It was introduced back in 2014, and was designed specifically for Apple's GPUs. Metal does provide support for some other GPUs, but because it was geared for Apple hardware it is different from OpenGL, Vulkan, or D3D12 and its predecessors.

In general, Metal does tessellation differently, and is missing geometry shaders and transform feedback. Specific to DirectX 12 and Metal, there is an issue with limits on resources. Generally, games need access to at least one million shader resource views (SRVs). Access to that many SRVs requires resource binding at the Tier 2 level. Metal only supports about 500,000 resources per argument buffer, so Tier 2 resource binding isn’t possible. Metal’s limit of half a million is sufficient for Vulkan descriptor indexing, but not for D3D12. This limitation means CrossOver Mac can't support Tier 2 binding and therefore a lot of DirectX 12 games will not run.

Another problem is that DirectX 12 uses GPU virtual addresses (VAs) to refer to resources for several things; most significantly ray tracing. According to Vulkan, their buffer device address (BDA) extension allows for the creation of complex data structures required for ray tracing, and useful for DirectX 12 porting. However, Apple has yet to add support for VAs or BDAs, insisting that existing argument buffer support is sufficient for what games want to do. While this may be technically true, it requires game designers to make a targeted effort to run on Metal. It is difficult for translation layers (i.e., MoltenVK or VKD3D) to support BDAs/GPU VAs on top of argument buffers, because argument buffers require you to encode the buffer reference into a separate argument buffer, which makes it more comparable to a Vulkan descriptor set or a DirectX 12 descriptor heap.

Considering these obstacles, we believe support for DirectX 12 on Mac could arrive as soon as CrossOver 23. Keep an eye out for updates, as we will be posting about the progress of DirectX 12 support on Crossover for Mac when we have news to share.

Cheers!

About Meredith Johnson
Meredith has been with CodeWeavers since 2019, and currently wears both the QA Manager and CrossOver Product Manager hats. After getting her PhD in linguistics, she somehow stumbled into the Free and Open Source Software world. When she isn't testing CrossOver, she's probably perfecting yet another bean stew, jogging slowly, buying even more linen tunics, doing a face mask or doting on her perfect chunky baby.