vsatp

Virtual Supervisor Address Translation and Protection

The vsatp register is a VSXLEN-bit read/write register that is VS-mode’s version of supervisor register satp. When V=1, vsatp substitutes for the usual satp, so instructions that normally read or modify satp actually access vsatp instead. vsatp controls VS-stage address translation, the first stage of two-stage translation for guest virtual addresses.

The vsatp register is considered active for the purposes of the address-translation algorithm unless the effective privilege mode is U and hstatus.HU=0. However, even when vsatp is active, VS-stage page-table entries' A bits must not be set as a result of speculative execution, unless the effective privilege mode is VS or VU.

In particular, virtual-machine load/store (hlv, 'hlvx', or 'hsv') instructions that are misspeculatively executed must not cause VS-stage A bits to be set.

When V=0, a write to vsatp with an unsupported MODE value is either ignored as it is for satp, or the fields of vsatp are treated as WARL in the normal way. However, when V=1, a write to satp with an unsupported MODE value is ignored and no write to vsatp is effected.

Attributes

CSR Address

0x280

Virtual CSR Address

0x180

Defining extension

  • H, version >= 0

Length

64-bit

Privilege Mode

VS

Format

vsatp format
Figure 1. vsatp format

Field Summary

Name Location Type Reset Value

MODE

63:60

RW-R

UNDEFINED_LEGAL

ASID

59:44

RW-R

UNDEFINED_LEGAL

PPN

43:0

RW-R

UNDEFINED_LEGAL

Fields

MODE

Location

vsatp[63:60]

Description

Translation Mode

Controls the current translation mode in VS-mode according to the table below.

Value

Name

Description +

0

Bare

No translation → virtual address == physical address

8

Sv39

39-bit virtual address translation

9

Sv48

48-bit virtual address translation

Any other value shall be ignored on a write.

Type

RW-R

Read-Write Restricted

Field is writable by software. Only certain values are legal. Writing an illegal value into the field is ignored, and the field retains its prior state.
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.

if (csr_value.MODE == 0) {
  if (virtual_mode?() || IGNORE_INVALID_VSATP_MODE_WRITES_WHEN_V_EQ_ZERO) {
    if (csr_value.ASID == 0 && csr_value.PPN == 0) {
      return csr_value.MODE;
    } else {
      return UNDEFINED_LEGAL_DETERMINISTIC;
    }
  } else {
    return UNDEFINED_LEGAL_DETERMINISTIC;
  }
} else if (true && csr_value.MODE == 8) {
  return csr_value.MODE;
} else if (true && csr_value.MODE == 9) {
  return csr_value.MODE;
} else {
  return UNDEFINED_LEGAL_DETERMINISTIC;
}

ASID

Location

vsatp[59:44]

Description

Address Space ID

Type

RW-R

Read-Write Restricted

Field is writable by software. Only certain values are legal. Writing an illegal value into the field is ignored, and the field retains its prior state.
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.

if (csr_value.MODE == 0) {
  if (virtual_mode?() || IGNORE_INVALID_VSATP_MODE_WRITES_WHEN_V_EQ_ZERO) {
    if (csr_value.ASID == 0 && csr_value.PPN == 0) {
      return csr_value.ASID;
    } else {
      return UNDEFINED_LEGAL_DETERMINISTIC;
    }
  } else {
    return UNDEFINED_LEGAL_DETERMINISTIC;
  }
} else {
  XReg shamt = 16;
  XReg all_ones = (0xffff);
  XReg largest_allowed_asid = 0xffff;
  if (csr_value.ASID == all_ones) {
    return largest_allowed_asid;
  } else if (csr_value.ASID > largest_allowed_asid) {
    return UNDEFINED_LEGAL_DETERMINISTIC;
  } else {
    return csr_value.ASID;
  }
}

PPN

Location

vsatp[43:0]

Description

Physical Page Number

The physical address of the active root page table is PPN << 12.

Can only hold values that correspond to a valid page table base, which will be implementation-dependent.

Type

RW-R

Read-Write Restricted

Field is writable by software. Only certain values are legal. Writing an illegal value into the field is ignored, and the field retains its prior state.
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.

if (csr_value.MODE == 0) {
  if (virtual_mode?() || IGNORE_INVALID_VSATP_MODE_WRITES_WHEN_V_EQ_ZERO) {
    if (csr_value.ASID == 0 && csr_value.PPN == 0) {
      return csr_value.PPN;
    } else {
      return UNDEFINED_LEGAL_DETERMINISTIC;
    }
  } else {
    return UNDEFINED_LEGAL_DETERMINISTIC;
  }
} else {
  return csr_value.PPN;
}