BizHawk
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TASEmulators
Waterbox & LLVMpipe
Since @nattthebear is non-trivial to catch on irc while I remember about this, I guess we need a ticket.
libTAS uses LLVMpipe to simulate hardware graphics stuff in software and make savestates of that.
Force software rendering
This option forces the OpenGL device driver Mesa 3D to use its software implementation of OpenGL (llvmpipe) to run the game. While this makes the game much slower, it is usually required to be able to use savestates. Indeed, the state of the GPU cannot be easily accessed and stored into savestates by the tool, thus savestates are incomplete and may crash the game. When using Mesa's software implementation, all of the graphics pipeline is done by the CPU and can be stored in the savestate.
You must be using Mesa 3D to be able to use this option, which usually means that you must be using the free driver for your GPU (e.g. nouveau for nvidia GPUs or radeon for ATI/AMD CPUs)
We know that waterbox can't snapshot GPU state, but would it be feasible to incorporate llvmpipe into waterboxhost so cores that require GPU (like Ruffle) could be ported?
Yes, this is a good idea. I've thought a bit about Mesa based rendering. LLVMPipe may be too advanced to support in the waterbox, but Mesa has other choices as well. I was always worried about speed; if it's glacial, then is it worth it?
I'd love to see speed as the only problem for some things that are otherwise outright impossible to get into hawk.
From a tas perspective, speed is of no concern as we tend to slow the game down anyways.
If it was like 1 frame every minute then it would be impractical.
If we're talking about arbitrary 3d cores like dolphin or citra, the speed very well may be that low. We can't assume anything about orders of magnitude here without research.
Both Dolphin and Citra have software renderers anyways. They would probably be faster than Mesa based rendering (assuming said renderers are optimized, which Dolphin definitely isn't anyways)
Ruffle (as mentioned) is the main thing that would greatly benefit from something like this, given it has no savestates but it doesn't have any true software renderer (although there is a separate question of can Rust code be waterboxed, but I imagine that is probably a yes with some work).
Rust is in principle waterboxable. How fast is regular ruffle with mesa?
Ruffle's speed is game dependent, but in libTAS with forced software rendering, I get about 0.5x-1x speed with Meat Boy and Super Mario Flash can get up to around 2x speed. Both Vulkan and GL run at around the same speed. This is not on a great computer either; I have Ubuntu running natively on an old laptop, it has these specs:
CPU: Intel® Core™ i5-4210M CPU @ 2.60GHz × 4 GPU: Intel® HD Graphics 4600 (HSW GT2)
LLVMpipe could be used for BlastEm core if you don't want to disable OpenGL?
Something to consider here (from LLVMpipe's page):
Also, the driver is multithreaded to take advantage of multiple CPU cores (up to 32 at this time).
Threads are junk in the box, so whatever speed measures you end up finding with libTAS + forced software rendering is likely going to be way better than what the box will give you.