[Mesa-dev] Landing ray-tracing support in ANV

[Iago Toral]

Hi Jason, this is really cool! Very happy to see that support for ray
tracing is in the works!

Iago

On Wed, 2020-10-28 at 17:26 -0500, Jason Ekstrand wrote:
> All,
> 
> Some of you may be curious about what I've been up to for most of
> 2020, why I've not been working on IBC, and why I suddenly decided to
> start caring about ray-tracing.  Well... it's all because I've been
> working on this little project to implement VK_KHR_ray_tracing in
> ANV.
> I finally have the approval to start talking publicly about the Intel
> ray-tracing implementation and can start landing code upstream.
> 
> To kick things off, I've posted a merge request which contains the
> bulk of the compiler work required to transform ray-tracing shaders
> into Intel's upcoming bindless dispatch model and compile them:
> 
> https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7356
> 
> This is only the first of what will likely be many MRs.  I've also
> created a tracking issue which will be used to help link it all
> together and keep me sane.  You can find it here:
> 
> https://gitlab.freedesktop.org/mesa/mesa/-/issues/3709
> 
> Apart from the core ray-tracing compiler bits, my next focus for
> upstream will be landing more OpenCL support patches.  In order to do
> GPU-side BVH building, I've got a pile of kernels written in OpenCL C
> that we need to be able to compile with our stack.  In order for
> those
> to work, we need a few more things:
> 
>  - Generic pointer support (!6332)
>  - SPV_INTEL_subgroups (!7145)
>  - ANV support for compiling and dispatching OpenCL kernels (coming
> soon)
>  - Misc NIR clean-ups and fixes.
> 
> The API bits for ray-tracing will be coming later.  I've got most of
> the API implemented but it's currently all written against the latest
> version of the KHR ray-tracing spec which hasn't been released
> publicly.  Once that spec goes public, I can start dropping patches.
> If the public KHR gets delayed too long and I run out of other bits
> to
> upstream, I may rebase the API bits on the provisional KHR from
> January or even the NV spec but I'd really rather not do that as it'd
> be a pile of churn in my internal branch for very little benefit.
> 
> LAQ (Likely Asked Questions):
> 
> Q: Do the patches work?  Have you tested them?
> A: Yes.  Don't worry, I'm not sending a bunch of untested garbage.
> Out of several thousand CTS tests, I've got 3 that are failing due to
> undiagnosed compiler bugs.  I'd say that's pretty good. :-)
> 
> Q: Will this work on older Intel GPUs?
> A: No.  While a pure SW emulation on top of compute shaders would be
> possible, this is not it.  This implementation requires new Xe HPG
> hardware that has not yet been released.
> 
> Q: Which upcoming Intel GPUs will support ray-tracing and when can I
> buy one?
> A: We’ve previously disclosed plans to release Xe HPG GPUs with
> dedicated hardware for ray tracing in 2021. I’m not authorized to
> tell
> you more. Sorry. Please don't ask. It just makes for an awkward
> conversation.
> 
> Q: Most of the new compiler code is in src/intel.  Can it be
> generalized for implementing ray-tracing in RADV or other Mesa Vulkan
> drivers?
> A: Yes, likely some of it can.  In particular, there is a shader
> splitting pass (for splitting shader calls into continuations) as
> well
> as a pass to inline any-hit shaders into intersection shaders.  With
> a
> little work, both of those could be generalized and pulled into
> src/compiler/nir.  The rest is mostly lowering code to translate to
> Intel's ray-tracing model.  Likely other hardware vendors models will
> be different enough that those passes won't be directly applicable.
> 
> Q: What about games that have already shipped ray-tracing support
> using VK_NV_ray_tracing?  Will you support those?
> A: I would eventually like to support any Vulkan games that support
> ray-tracing.  However, it's still a bit TBD just how we'll deal with
> games that use the NV extension.  Ideally, once VK_KHR_ray_tracing
> ships publicly, we would convince the game vendors who have already
> shipped ray-tracing support to push an update that switches their
> game
> over to the KHR extension.  If that fails, it would be pretty easy to
> write a layer which translates from the NV extension to the KHR one.
> 
> That's all for now, folks!  I'll be on IRC and e-mail if you have any
> questions.  I'd love to get some outside review on the compiler
> patches if I could.  The shader splitting pass, in particular, is a
> bit on the gnarley side.
> 
> --Jason
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