That's not really what I meant. It's very easy to configure which instructions or CSRs exist, and excluding custom extensions there are really only two options

That's not really what I meant. It's very easy to configure which instructions or CSRs exist, and excluding custom extensions there are really only two options for behaviour, so you just have a flag `haveSomeExtension()` to enable the instructions and CSRs. The Sail model has some of these flags already. If you write to a WARL field in a CSR the chip can use more or less any legalisation logic it wants. Configuring that is very difficult (though there is a decent attempt in riscv-config).