mstateen0

Machine State Enable 0 Register

Each bit of a stateen CSR controls less-privileged access to an extension’s state, or an extension that was not deemed "worthy" of a full XS field in sstatus like the FS and VS fields for the F and V extensions.

The number of registers provided at each level is four because it is believed that 4 * 64 = 256 bits for machine and hypervisor levels, and 4 * 32 = 128 bits for supervisor level, will be adequate for many years to come, perhaps for as long as the RISC-V ISA is in use. The exact number four is an attempted compromise between providing too few bits on the one hand and going overboard with CSRs that will never be used on the other.

The stateen registers at each level control access to state at all less-privileged levels, but not at its own level.

When a stateen CSR prevents access to state for a privilege mode, attempting to execute in that privilege mode an instruction that implicitly updates the state without reading it may or may not raise an illegal instruction or virtual instruction exception. Such cases must be disambiguated by being explicitly specified one way or the other. In some cases, the bits of the stateen CSRs will have a dual purpose as enables for the ISA extensions that introduce the controlled state.

For every bit with a defined purpose in an sstateen CSR, the same bit is defined in the matching mstateen CSR to control access below machine level to the same state. The upper 32 bits of an mstateen CSR (or for RV32, the corresponding high-half CSR) control access to state that is inherently inaccessible to user level, so no corresponding enable bits in the supervisor-level sstateen CSR are applicable. The intention is to allocate bits for this purpose starting at the most-significant end, bit 63, through to bit 32, and then on to the next-higher mstateen CSR. If the rate that bits are being allocated from the least-significant end for sstateen CSRs is sufficiently low, allocation from the most-significant end of mstateen CSRs may be allowed to encroach on the lower 32 bits before jumping to the next-higher mstateen CSR. In that case, the bit positions of "encroaching" bits will remain forever read-only zeros in the matching sstateen CSRs.

For every bit in an mstateen CSR that is zero (whether read-only zero or set to zero), the same bit appears as read-only zero in the matching hstateen and sstateen CSRs.

A bit in a supervisor-level sstateen CSR cannot be read-only one unless the same bit is read-only one in the matching mstateen CSR and, if it exists, in the matching hstateen CSR. A bit in an hstateen CSR cannot be read-only one unless the same bit is read-only one in the matching mstateen CSR. Bit 63 of each mstateen CSR may be read-only zero only if the hypervisor extension is not implemented and the matching supervisor-level sstateen CSR is all read-only zeros.