EL0 Read/Write Software Context Number
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) and FEAT_AA64 is implemented. Otherwise, direct accesses to SCXTNUM_EL0 are UNDEFINED.
SCXTNUM_EL0 is a 64-bit register.
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| SCXTNUM | |||||||||||||||||||||||||||||||
| SCXTNUM | |||||||||||||||||||||||||||||||
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 = 0b11, op1 = 0b011, CRn = 0b1101, CRm = 0b0000, op2 = 0b111)
if !((IsFeatureImplemented(FEAT_CSV2_2) || IsFeatureImplemented(FEAT_CSV2_1p2)) && IsFeatureImplemented(FEAT_AA64)) then Undefined(); elsif HaveEL(EL3) && PSTATE.EL != EL3 && EL3SDDUndefPriority() && SCR_EL3().EnSCXT == '0' then Undefined(); elsif PSTATE.EL == EL0 then if EffectivelyAtEL0NotInHost() && SCTLR_EL1().TSCXT == '1' then AArch64_SystemAccessTraptoEL1orEL2(0x18); elsif EL2Enabled() && EffectivelyAtEL0NotInHost() && HCR_EL2().EnSCXT == '0' then AArch64_SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && EffectivelyAtEL0NotInHost() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3().FGTEn == '1') && HFGRTR_EL2().SCXTNUM_EL0 == '1' then AArch64_SystemAccessTrap(EL2, 0x18); elsif EffectivelyAtEL0InHost() && 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); end; else X{64}(t) = SCXTNUM_EL0(); end; elsif PSTATE.EL == EL1 then if 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); end; else X{64}(t) = SCXTNUM_EL0(); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && SCR_EL3().EnSCXT == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else X{64}(t) = SCXTNUM_EL0(); end; elsif PSTATE.EL == EL3 then X{64}(t) = SCXTNUM_EL0(); end;
MSR SCXTNUM_EL0, <Xt>
(op0 = 0b11, op1 = 0b011, CRn = 0b1101, CRm = 0b0000, op2 = 0b111)
if !((IsFeatureImplemented(FEAT_CSV2_2) || IsFeatureImplemented(FEAT_CSV2_1p2)) && IsFeatureImplemented(FEAT_AA64)) then Undefined(); elsif HaveEL(EL3) && PSTATE.EL != EL3 && EL3SDDUndefPriority() && SCR_EL3().EnSCXT == '0' then Undefined(); elsif PSTATE.EL == EL0 then if EffectivelyAtEL0NotInHost() && SCTLR_EL1().TSCXT == '1' then AArch64_SystemAccessTraptoEL1orEL2(0x18); elsif EL2Enabled() && EffectivelyAtEL0NotInHost() && HCR_EL2().EnSCXT == '0' then AArch64_SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && EffectivelyAtEL0NotInHost() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3().FGTEn == '1') && HFGWTR_EL2().SCXTNUM_EL0 == '1' then AArch64_SystemAccessTrap(EL2, 0x18); elsif EffectivelyAtEL0InHost() && 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); end; else SCXTNUM_EL0() = X{64}(t); end; elsif PSTATE.EL == EL1 then if 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); end; else SCXTNUM_EL0() = X{64}(t); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && SCR_EL3().EnSCXT == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x18); end; else SCXTNUM_EL0() = X{64}(t); end; elsif PSTATE.EL == EL3 then SCXTNUM_EL0() = X{64}(t); end;
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