Proton is getting some "horrible" workarounds for Forza Horizon 6 on Linux

According to the developer of VKD3D-Proton, Forza Horizon 6 is "extremely broken" and needs some "horrible" workarounds to run properly on Linux. It's due for release tomorrow, after being in Advanced Access since just before the weekend.

With the advanced access many Linux / SteamOS and Steam Deck gamers have been jumping in, but the experience has been seemingly quite a mixed bag.

Writing in a GitHub draft post, developer Hans-Kristian Arntzen who works on VKD3D-Proton (the Direct3D 12 -> Vulkan layer Proton uses) mentioned a lot of issues with the game. Various reports on ProtonDB also show people having lots of issues with the game across both PC and Steam Deck. So you may want to hold off on this until there's some official updates.

From the GitHub post:

This game is extremely broken:

This pulls in the rest of descriptor heap, since it's only reasonable way right now to get working benchmark without rendering glitches on my system. At best, this can be a hotfix branch or something.

Needs more investigation why the 32 + 32 RDNA2 style path doesn't quirk work. But it doesn't hang GPU anymore, so ... yay?

Adds sampler heap robustness too in dxil-spirv by just padding all sampler heaps to the 2048 limit. There is no room to pass down sampler heap size, so just clamp to constant 2047 instead, which is fine in practice. There are no null samplers, so we'll use whatever is there I guess when it's OOB.

Another issue is missing nonuniformEXT in RT shaders which I added force robustness for RT only to avoid crippling unrelated shaders. I only detected issues in this area in RT.

After that, we have the "lovely" bug of bad aliasing between image descriptors and buffer descriptors. The game first writes an image descriptor, then reads that as a plain buffer, which is broken, and on RDNA3/4 we would read the image descriptor at a 16b sliced offset, which breaks. This is the first instance of this causing issues in the wild. It was a known hazard point, but we have easy mitigation strategies on AMD at least.

Fixing this for descriptor buffer is very hard since it means rejigging new fast paths for what is supposed to be a really terrible workaround. The workaround for now only applies to descriptor heap since it was quite easy to do.

There are also bugs with "use-before-alloc" where the application allocates resources after submit and writing descriptors pointing to them to the descriptor heap while GPU is in flight. Utter insanity, but FH5 had issues in this area too.

It's not been proven to cause a hang yet I think, but radv_wait_for_vm_map_updates=true is very likely going to be needed for safety.

RADV_EXPERIMENTAL=heap is also needed since descriptor heap is not enabled by default.

Also fixes a silly bug that happened with RDNA2 descriptor heap while dealing with reviews, but that's not really relevant to this PR.

tl;dr:

• This PR
• Use Mesa git
• RADV_EXPERIMENTAL=heap radv_wait_for_vm_map_updates=true %command%

Should get the game through the benchmark at least ... Haven't tested much ingame so far.

We may see Valve put out a Proton Hotfix specifically for Forza Horizon 6, or they might just bump up some upgrades directly into Proton Experimental for it. A few hours ago Valve did just tag the game to use Proton Experimental.

This goes to show at times what a difficult a job it is that Valve developers have. Since Linux / SteamOS is still a relatively low user share on Steam, developers are not ensuring their games are compatible. Valve often have to make special fixes for the bigger releases to get them working. Hopefully the likes of the Steam Machine, Steam Frame and eventually a Steam Deck 2 will keep pumping up those numbers of users on Linux.

I haven't been able to test directly, as no one from Playground Games replied to our request to cover it.