vsireg2 is a VS indirect-CSR access CSR. The separate CSR address is for M/HS-side management; when V=1, guest access to the corresponding supervisor CSR is substituted with VS state unless an extension specifies otherwise.
vsireg2 address, access class, and width are checked against the official CSR tables: 0x252, HRW, current XLEN, not VSXLEN.
vsiselect supports at least selector range 0..0xFFF; custom selectors use the space with the most-significant bit set.
Direct VS/VU access to vsiselect/vsireg* raises a virtual-instruction exception; a guest normally uses the siselect/sireg* aliases.
When writing, modify only officially defined fields; handle WARL, WLRL, WPRI, and reserved fields according to the official specification and implementation behavior.