Hi. I’m the guy responsible for PolkaVM. Since I see some misinformation flying around here and there let me chime in with some details as to what PolkaVM is and how it works.
PolkaVM currently supports riscv64emac with the Zbb extension, but unlike most (all?) other RISC-V VMs it doesn’t run RISC-V binaries as-is (it’s not actually a RISC-V VM!). Offline we ingest normal RISC-V ELF binaries that you build using a normal compiler, and we translate them into a more constrained and efficient custom bytecode that’s designed for use in a VM (and not native hardware, like RISC-V is). The idea is to remove as much complexity out of the VM itself (which needs to run on-chain), put as much of that complexity as possible into offline tooling (which can run off-chain), and improve security by removing unnecessary features (e.g. in vanilla RISC-V you can jump to any address; in PolkaVM bytecode the code address space is fully virtualized, you can’t jump everywhere, and the bytecode isn’t even loaded into memory that’s accessible by the program).
Performance-wise we get very close to bare metal performance[1]; it’s as fast as wasmtime (which is the state-of-art WASM VM), but it guarantees O(n) recompilation and recompiles programs into native code hundreds times faster. To put this into perspective, it is faster to recompile a program from scratch starting with raw PolkaVM bytecode into the native code than it is to cache the recompilation artifacts and look them up based on their hash (in other words, recompiling the program is faster than calculating its hash), and this is without sacrificing runtime execution performance.
We mainly use RISC-V not because it’s a particularly good bytecode for a VM (it actually isn’t that great), but because it’s simple, well supported and it’s relatively easy to translate to something else, so we can get the best of both worlds - great software compatibility (you can use existing compilers and programming languages, e.g. the other day I ported Quake to PolkaVM[2][3]), but you also get the benefits of a custom, optimized bytecode (blazing fast compilation speed, near-native performance, simplicity and customizability).
Anyway, if you have any questions feel free to ask me anything.
Links: (New account so I’m not allowed to post links, so just replace “github” with github dot com)
[1] - github/paritytech/polkavm/blob/master/BENCHMARKS.md
[2] - github/paritytech/polkaports/tree/master/apps/quake
[3] - github/paritytech/polkavm/tree/master/examples/quake