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SCXTNUM_EL2, EL2 Read/Write Software Context Number

The SCXTNUM_EL2 characteristics are:

Purpose

Provides a number that can be used to separate out different context numbers with the EL2 exception level, for the purpose of protecting against side-channels using branch prediction and similar resources.

Configuration

This register is present only when ARMv8.0-CSV2 is implemented. Otherwise, direct accesses to SCXTNUM_EL2 are UNDEFINED.

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

SCXTNUM_EL2 is a 64-bit register.

Field descriptions

The SCXTNUM_EL2 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
Software Context Number
Software Context Number
313029282726252423222120191817161514131211109876543210

Bits [63:0]

Software Context Number. A number to identify the context within the EL2 exception level.

This field resets to an architecturally UNKNOWN value.

Accessing the SCXTNUM_EL2

When HCR_EL2.E2H is 1, without explicit synchronization, access from EL2 using the mnemonic SCXTNUM_EL2 or SCXTNUM_EL1 are not guaranteed to be ordered with respect to accesses using the other mnemonic.

Accesses to this register use the following encodings:

MRS <Xt>, SCXTNUM_EL2

CRnop0op1op2CRm
0b11010b110b1000b1110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        return SCXTNUM_EL2;
elsif PSTATE.EL == EL3 then
    return SCXTNUM_EL2;
              

MSR SCXTNUM_EL2, <Xt>

CRnop0op1op2CRm
0b11010b110b1000b1110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        SCXTNUM_EL2 = X[t];
elsif PSTATE.EL == EL3 then
    SCXTNUM_EL2 = X[t];
              

MRS <Xt>, SCXTNUM_EL1

CRnop0op1op2CRm
0b11010b110b0000b1110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<NV2,NV1,NV> == '111' then
        return NVMem[0x188];
    else
        return SCXTNUM_EL1;
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    elsif HCR_EL2.E2H == '1' then
        return SCXTNUM_EL2;
    else
        return SCXTNUM_EL1;
elsif PSTATE.EL == EL3 then
    return SCXTNUM_EL1;
              

MSR SCXTNUM_EL1, <Xt>

CRnop0op1op2CRm
0b11010b110b0000b1110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<NV2,NV1,NV> == '111' then
        NVMem[0x188] = X[t];
    else
        SCXTNUM_EL1 = X[t];
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    elsif HCR_EL2.E2H == '1' then
        SCXTNUM_EL2 = X[t];
    else
        SCXTNUM_EL1 = X[t];
elsif PSTATE.EL == EL3 then
    SCXTNUM_EL1 = X[t];
              


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