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

Requirement

Smcsrind

Defining extensions

Smcsrind

Machine-mode Indirect CSR Access

CSR Address

0x251

Virtual CSR Address

0x251

Length

* 32 when CSR[hstatus].VSXL == 0 * 64 when CSR[hstatus].VSXL == 1

Privilege Mode

VS

Format

This CSR format changes dynamically.

vsireg Format when CSR[hstatus].VSXL == 0
Figure 1. vsireg Format when CSR[hstatus].VSXL == 0
vsireg Format when CSR[hstatus].VSXL == 1
Figure 2. vsireg Format when CSR[hstatus].VSXL == 1

Field Summary

Name Location Type Reset Value

vsireg.VALUE

* 31:0 when CSR[hstatus].VSXL == 0 * 63:0 when CSR[hstatus].VSXL == 1

RW

UNDEFINED_LEGAL

Fields

VALUE

Location

  • 31:0 when CSR[hstatus].VSXL == 0

  • 63:0 when CSR[hstatus].VSXL == 1

Description

The data read from or written to the register selected by the current value of the vsiselect CSR.

Type

RW

Reset value

UNDEFINED_LEGAL

Software write

This CSR may store a value that is different from what software attempts to write.

When a software write occurs (e.g., through csrrw), the following determines the written value:

VALUE = Csr handle = indirect_csr_lookup(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.

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