RISC-V Extensions query for QEMU System

I have a query regarding which RISC-V extensions are available in QEMU System Emulator (especially in generic virt machine). On the documentation here, it mentions Up to 512 generic RV32GC/RV64GC cores, with optional extensions. I would like to know which extensions are "optional".

It would be great if you point me to the page where it is written specifically that extensions are supported in RISC-V.