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SCXTNUM_EL0, EL0 Read/Write Software Context Number

The SCXTNUM_EL0 characteristics are:

Purpose

Provides a number that can be used to separate out different context numbers with the EL0 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_EL0 are UNDEFINED.

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

SCXTNUM_EL0 is a 64-bit register.

Field descriptions

The SCXTNUM_EL0 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 EL0 exception level.

This field resets to an architecturally UNKNOWN value.

Accessing the SCXTNUM_EL0

Accesses to this register use the following encodings:

MRS <Xt>, SCXTNUM_EL0

op0op1CRnCRmop2
0b110b0110b11010b00000b111
if PSTATE.EL == EL0 then
    if !ELUsingAArch32(EL1) && !(EL2Enabled() && HCR_EL2.<E2H,TGE> == '11') && SCTLR_EL1.TSCXT == '1' then
        if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TGE == '1' then
            AArch64.SystemAccessTrap(EL2, 0x18);
        else
            AArch64.SystemAccessTrap(EL1, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> != '11' && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL1) && HCR_EL2.<E2H,TGE> != '11' && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGRTR_EL2.SCXTNUM_EL0 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> == '11' && SCTLR_EL2.TSCXT == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        return SCXTNUM_EL0;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGRTR_EL2.SCXTNUM_EL0 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        return SCXTNUM_EL0;
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        return SCXTNUM_EL0;
elsif PSTATE.EL == EL3 then
    return SCXTNUM_EL0;
              

MSR SCXTNUM_EL0, <Xt>

op0op1CRnCRmop2
0b110b0110b11010b00000b111
if PSTATE.EL == EL0 then
    if !ELUsingAArch32(EL1) && !(EL2Enabled() && HCR_EL2.<E2H,TGE> == '11') && SCTLR_EL1.TSCXT == '1' then
        if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.TGE == '1' then
            AArch64.SystemAccessTrap(EL2, 0x18);
        else
            AArch64.SystemAccessTrap(EL1, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> != '11' && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL1) && HCR_EL2.<E2H,TGE> != '11' && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGWTR_EL2.SCXTNUM_EL0 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.<E2H,TGE> == '11' && SCTLR_EL2.TSCXT == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        SCXTNUM_EL0 = X[t];
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HCR_EL2.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif EL2Enabled() && !ELUsingAArch32(EL2) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGWTR_EL2.SCXTNUM_EL0 == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    elsif HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        SCXTNUM_EL0 = X[t];
elsif PSTATE.EL == EL2 then
    if HaveEL(EL3) && !ELUsingAArch32(EL3) && SCR_EL3.EnSCXT == '0' then
        AArch64.SystemAccessTrap(EL3, 0x18);
    else
        SCXTNUM_EL0 = X[t];
elsif PSTATE.EL == EL3 then
    SCXTNUM_EL0 = X[t];
              


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