vsireg

Virtual Supervisor Indirect Register Alias

The vsireg CSR is one of several alias registers used to indirectly access virtual supervisor-level CSRs in VS-mode or VU-mode.

The register addressed by vsireg is selected by the current value of the vsiselect CSR.

The alias mechanism allows indirect CSR access, which helps in virtualization and future extensibility.

A virtual instruction exception is raised for attempts from VS-mode or VU-mode to directly access vsiselect or vsireg, or from VU-mode to access siselect or sireg.

The behavior of accesses to vsireg when vsiselect holds a value that is not implemented at the HS level is UNSPECIFIED.

Implementations are recommended to raise an illegal instruction exception for accesses to unimplemented targets via vsireg.

The width of vsireg is always the current XLEN, not VSXLEN. For example, if HSXLEN = 64 and VSXLEN = 32, then vsireg is 64 bits wide when accessed from HS-mode (RV64), but 32 bits when accessed from VS-mode (RV32).

Attributes

CSR Address	0x251
Virtual CSR Address	0x251
Defining extension	Smcsrind
Length	64-bit
Privilege Mode	VS

CSR Address

0x251

Virtual CSR Address

0x251

Defining extension

Smcsrind

Length

64-bit

Privilege Mode

Format

Figure 1. vsireg format

Field Summary

Name Location Type Reset Value

Name	Location	Type	Reset Value
`VALUE`	63:0	RW	UNDEFINED_LEGAL

VALUE

63:0

UNDEFINED_LEGAL

Fields

`VALUE`

Location: vsireg[63:0]
Description: The data read from or written to the register selected by the current value of the vsiselect CSR.
Type

RW	Read-Write Field is writable by software. Any value that fits in the field is acceptable and shall be retained for subsequent reads.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

Csr handle = indirect_csr_lookup(PrivilegeMode::VS, CSR[vsiselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
if (!handle.writable) {
  raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
csr_sw_write(handle, csr_value.VALUE);
return csr_hw_read(handle);

Software read

This CSR may return a value that is different from what is stored in hardware.

Pruned
Original

Csr handle = indirect_csr_lookup(PrivilegeMode::VS, CSR[vsiselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);

Csr handle = indirect_csr_lookup(PrivilegeMode::VS, CSR[vsiselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);