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.
This register is present only when FEAT_CSV2_2 is implemented or FEAT_CSV2_1p2 is implemented. Otherwise, direct accesses to SCXTNUM_EL0 are UNDEFINED.
SCXTNUM_EL0 is a 64-bit register.
| 63 | 62 | 61 | 60 | 59 | 58 | 57 | 56 | 55 | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 40 | 39 | 38 | 37 | 36 | 35 | 34 | 33 | 32 |
| 31 | 30 | 29 | 28 | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Software Context Number | |||||||||||||||||||||||||||||||
| Software Context Number | |||||||||||||||||||||||||||||||
Software Context Number. A number to identify the context within the EL0 exception level.
The reset behavior of this field is:
Accesses to this register use the following encodings in the System register encoding space:
MRS <Xt>, SCXTNUM_EL0
| op0 | op1 | CRn | CRm | op2 |
|---|---|---|---|---|
| 0b11 | 0b011 | 0b1101 | 0b0000 | 0b111 |
if PSTATE.EL == EL0 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif !ELIsInHost(EL0) && SCTLR_EL1.TSCXT == '1' then if EL2Enabled() && HCR_EL2.TGE == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.SystemAccessTrap(EL1, 0x18); elsif EL2Enabled() && !ELIsInHost(EL0) && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && !ELIsInHost(EL0) && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGRTR_EL2.SCXTNUM_EL0 == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif ELIsInHost(EL0) && SCTLR_EL2.TSCXT == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else X[t, 64] = SCXTNUM_EL0; elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif EL2Enabled() && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGRTR_EL2.SCXTNUM_EL0 == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else X[t, 64] = SCXTNUM_EL0; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else X[t, 64] = SCXTNUM_EL0; elsif PSTATE.EL == EL3 then X[t, 64] = SCXTNUM_EL0;
MSR SCXTNUM_EL0, <Xt>
| op0 | op1 | CRn | CRm | op2 |
|---|---|---|---|---|
| 0b11 | 0b011 | 0b1101 | 0b0000 | 0b111 |
if PSTATE.EL == EL0 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif !ELIsInHost(EL0) && SCTLR_EL1.TSCXT == '1' then if EL2Enabled() && HCR_EL2.TGE == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.SystemAccessTrap(EL1, 0x18); elsif EL2Enabled() && !ELIsInHost(EL0) && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && !ELIsInHost(EL0) && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGWTR_EL2.SCXTNUM_EL0 == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif ELIsInHost(EL0) && SCTLR_EL2.TSCXT == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t, 64]; elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif EL2Enabled() && HCR_EL2.EnSCXT == '0' then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGWTR_EL2.SCXTNUM_EL0 == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t, 64]; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && SCR_EL3.EnSCXT == '0' then UNDEFINED; elsif HaveEL(EL3) && SCR_EL3.EnSCXT == '0' then if EL3SDDUndef() then UNDEFINED; else AArch64.SystemAccessTrap(EL3, 0x18); else SCXTNUM_EL0 = X[t, 64]; elsif PSTATE.EL == EL3 then SCXTNUM_EL0 = X[t, 64];