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VTTBR_EL2, Virtualization Translation Table Base Register

The VTTBR_EL2 characteristics are:

Purpose

Holds the base address of the translation table for the initial lookup for stage 2 of an address translation in the EL1&0 translation regime, and other information for this translation regime.

Configuration

AArch64 System register VTTBR_EL2 bits [63:0] are architecturally mapped to AArch32 System register VTTBR[63:0] .

If EL2 is not implemented, this register is RES0 from EL3.

This register has no effect if EL2 is not enabled in the current Security state.

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

VTTBR_EL2 is a 64-bit register.

Field descriptions

The VTTBR_EL2 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
VMID[15:8]VMID[7:0]BADDR
BADDRCnP
313029282726252423222120191817161514131211109876543210

VMID[15:8], bits [63:56]

When ARMv8.1-VMID16 is implemented:

Extension to VMID[7:0]. See VMID[7:0] for more details.

This field resets to an architecturally UNKNOWN value.


Otherwise:

Reserved, RES0.

VMID[7:0], bits [55:48]

The VMID for the translation table.

It is IMPLEMENTATION DEFINED whether the VMID is 8 bits or 16 bits.

If the implementation has an 8-bit VMID, then VMID[15:8] are RES0.

If the implementation has a 16-bit VMID, then:

  • The VTCR_EL2.VS bit selects whether VMID[15:8] are ignored by the hardware for every purpose except reading back the register, or whether these bits are used for allocation and matching in the TLB.
  • The 16-bit VMID is only supported when EL2 is using AArch64. This means the hardware must ignore VMID[15:8] when EL2 is using AArch32.

This field resets to an architecturally UNKNOWN value.

BADDR, bits [47:1]

Translation table base address, A[47:x] or A[51:x], bits[47:1].

Note
  • Translation table base addresses of 52 bits, A[51:x], are supported only in an implementation that includes ARMv8.2-LPA and is using the 64KB translation granule.
  • A translation table must be aligned to the size of the table, except that when using a translation table base address larger than 48 bits the minimum alignment of a table containing fewer than eight entries is 64 bytes.

In an implementation that includes ARMv8.2-LPA, if the value of VTCR_EL2.PS is 0b110, then:

  • Register bits[47:z] hold bits[47:z] of the stage 1 translation table base address, where z is determined as follows:
    • If x >= 6 then z=x.
    • Otherwise, z=6.
  • Register bits[5:2] hold bits[51:48] of the stage 1 translation table base address.
  • When z>x register bits[(z-1):x] are RES0, and bits[(z-1):x] of the translation table base address are zero.
  • When x>6 register bits[(x-1):6] are RES0.
  • Register bit[1] is RES0.
  • Bits[5:2] of the stage 1 translation table base address are zero.
  • In an implementation that includes ARMv8.2-TTCNP, bit[0] of the stage 1 translation table base address is zero.
Note
  • In an implementation that includes ARMv8.2-LPA a VTCR_EL2.PS value of 0b110, that selects a PA size of 52 bits, is permitted only when using the 64KB translation granule.
  • When the value of ID_AA64MMFR0_EL1.PARange indicates that the implementation does not support a 52 bit PA size, if a translation table lookup uses this register with the 64KB translation granule when the value of VTCR_EL2.PS is 0b110 and the value of register bits[5:2] is nonzero, an Address size fault is generated.

If the Effective value of VTCR_EL2.PS is not 0b110 then:

  • Register bits[47:x] hold bits[47:x] of the stage 1 translation table base address.
  • Register bits[(x-1):1] are RES0.
  • If the implementation supports 52-bit PAs and IPAs then bits[51:48] of the translation table base addresses used in this stage of translation are 0b0000.
Note

This definition applies:

  • To an implementation that includes ARMv8.2-LPA and is using a translation granule smaller than 64KB.
  • To any implementation that does not include ARMv8.2-LPA.

If any VTTBR_EL2[47:0] bit that is defined as RES0 has the value 1 when a translation table walk is performed using VTTBR_EL2, then the translation table base address might be misaligned, with effects that are CONSTRAINED UNPREDICTABLE, and must be one of the following:

  • Bits[x-1:0] of the translation table base address are treated as if all the bits are zero. The value read back from the corresponding register bits is either the value written to the register or zero.
  • The result of the calculation of an address for a translation table walk using this register can be corrupted in those bits that are nonzero.

The AArch64 Virtual Memory System Architecture chapter describes how x is calculated based on the value of VTCR_EL2.T0SZ, the stage of translation, and the translation granule size.

This field resets to an architecturally UNKNOWN value.

CnP, bit [0]

When ARMv8.2-TTCNP is implemented:

Common not Private. This bit indicates whether each entry that is pointed to by VTTBR_EL2 is a member of a common set that can be used by every PE in the Inner Shareable domain for which the value of VTTBR_EL2.CnP is 1.

CnPMeaning
0b0

The translation table entries pointed to by VTTBR_EL2 are permitted to differ from the entries for VTTBR_EL2 for other PEs in the Inner Shareable domain. This is not affected by the value of the current VMID.

0b1

The translation table entries pointed to by VTTBR_EL2 are the same as the translation table entries for every other PE in the Inner Shareable domain for which the value of VTTBR_EL2.CnP is 1 and the VMID is the same as the current VMID.

This field is permitted to be cached in a TLB.

Note

If the value of VTTBR_EL2.CnP bit is 1 on multiple PEs in the same Inner Shareable domain and those VTTBR_EL2s do not point to the same translation table entries when using the current VMID then the results of translations using VTTBR_EL2 are CONSTRAINED UNPREDICTABLE, see 'CONSTRAINED UNPREDICTABLE behaviors due to caching of control or data values' in the Arm® Architecture Reference Manual, Armv8, for Armv8-A architecture profile.

This field resets to an architecturally UNKNOWN value.


Otherwise:

Reserved, RES0.

Accessing the VTTBR_EL2

Accesses to this register use the following encodings:

MRS <Xt>, VTTBR_EL2

op0op1CRnCRmop2
0b110b1000b00100b00010b000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.<NV2,NV> == '11' then
        return NVMem[0x020];
    elsif EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    return VTTBR_EL2;
elsif PSTATE.EL == EL3 then
    return VTTBR_EL2;
              

MSR VTTBR_EL2, <Xt>

op0op1CRnCRmop2
0b110b1000b00100b00010b000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.<NV2,NV> == '11' then
        NVMem[0x020] = X[t];
    elsif EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    VTTBR_EL2 = X[t];
elsif PSTATE.EL == EL3 then
    VTTBR_EL2 = X[t];
              


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