Note that the base RISC-V ISA (IMA extensions) doesn't say anything about calling conventions or functions (calls and returns) or even a stack. All X registers

Note that the base RISC-V ISA (IMA extensions) doesn't say anything about calling conventions or functions (calls and returns) or even a stack. All X registers except X0 are interchangeable. Which registers are caller save or callee save or even which is the stack pointer or function return address is all purely a software convention in a particular OS and/or compiler. (the C extension gives greater program compression if used with an ABI that uses X1 as the Return Address and X2 as the Stack Pointer, and X8-X15 as the most commonly used registers, but even then nothing breaks if you don't)