If you just want to test the idea without any major complexity, like "I just want to add an instruction for int8 dot products" or whatever, you can write your o

If you just want to test the idea without any major complexity, like "I just want to add an instruction for int8 dot products" or whatever, you can write your own emulator for RV32IMC, in M-mode, in a few hundred lines of most modern programming languages. There are a bunch of test suites to check the base ISA works, and you can use a C compiler to target bare metal, and use GCC's .insn directive to embed custom opcodes inline. Run a bunch of tests this way. It's actually very easy to control the whole stack in cases like this. Once you get into actually needing to boot a system like Linux, you're going to need more support from the emulator, and at that level you'd either want to look into extending the Spike emulator, or QEMU itself. Like, features that interact with different privilege levels or whatever are just complex, there isn't any way around just working on the source directly. I realize that's not a very good answer. There are simpler kernels and whatnot you could also probably use. I mean, I would hope so. Honestly, a lot of it depends on the specifics of what you're looking at trying to implement. Simple instructions that work in M/U mode are the best place to start. I suggest trying to write your own emulator first. It'd surprise you at how easy the base RISC-V IMC ISA is, and you can add as many custom instructions as you like, and you'll have a C compiler (and Rust too, actually) to start with.