The spec is free, and a lot of cores are free as well. Here's the HDL for AliBaba's cores they've been developing this against: https://github.com/T-head-Semi/

The spec is free, and a lot of cores are free as well. Here's the HDL for AliBaba's cores they've been developing this against: https://github.com/T-head-Semi/openc910 https://github.com/T-head-Semi/openc910 There are also non-free RISC-V cores, but they essentially have to continuously make the argument for their existence against the free cores that are available (support, verification resources, just being better designs in the short term, already being hardened for proprietary process nodes, etc.)