SMCR_EL1 SME Control Register (EL1) when FEAT_SME is implemented This register controls aspects of Streaming SVE that are visible at Exception levels EL1 and EL0. Other This register has no effect if the PE is not in Streaming SVE mode. When the Effective value of HCR_EL2.{E2H, TGE} is {1, 1}, this register has no effect on execution at EL0. SMCR_EL1 is a 64-bit register. 63 32 63:32 Reserved, RES0. FA64 31 31 0 Controls whether execution of an A64 instruction at EL1 is considered legal when executed in Streaming SVE mode. When the Effective value of HCR_EL2.{E2H, TGE} is not {1, 1}, controls whether execution of an A64 instruction at EL0 is considered legal when executed in Streaming SVE mode. Arm recommends that portable SME software should not rely on this optional feature, and that operating systems should provide a means to test for compliance with this recommendation. 0b0 This control does not cause any instruction to be treated as legal when executed in Streaming SVE mode. 0b1 This control causes all implemented A64 instructions to be treated as legal when executed in Streaming SVE mode at EL1 and EL0, if they are treated as legal at more privileged Exception levels in the current Security state. AU When FEAT_SME_FA64 is implemented 31 31 31 Reserved, RES0. Otherwise EZT0 30 30 0 Traps execution at EL1 and EL0 of the LDR, LUTI2, LUTI4, MOVT, STR, and ZERO instructions that access the ZT0 register to EL1, or to EL2 when EL2 is implemented and enabled in the current Security state and HCR_EL2.TGE is 1. The exception is reported using ESR_EL1.EC or ESR_EL2.EC value 0x1D, with an ISS code of 0x0000004, at a lower priority than a trap due to PSTATE.SM or PSTATE.ZA. Changes to this field only affect whether instructions that access ZT0 are trapped. They do not affect the contents of ZT0, which remain valid so long as PSTATE.ZA is 1. 0b0 This control causes execution of these instructions at EL1 and EL0 to be trapped. 0b1 This control does not cause execution of any instruction to be trapped. AU When FEAT_SME2 is implemented 30 30 30 Reserved, RES0. Otherwise 29 9 29:9 Reserved, RES0. 8 4 8:4 Reserved, RAZ/WI. LEN 3 0 3:0 Requests an Effective Streaming SVE vector length (SVL) at EL1 of (LEN+1)*128 bits. This field also defines the Effective Streaming SVE vector length at EL0 when EL2 is not implemented, or EL2 is not enabled in the current Security state, or the Effective value of HCR_EL2.{E2H, TGE} is not {1, 1}. The Streaming SVE vector length can be any power of two from 128 bits to 2048 bits inclusive. An implementation can support any subset of the architecturally permitted lengths. When the PE is in Streaming SVE mode, the Effective SVE vector length (VL) is equal to SVL. When FEAT_SVE is implemented, and the PE is not in Streaming SVE mode, VL is equal to the Effective Non-streaming SVE vector length. See ZCR_EL1. For all purposes other than returning the result of a direct read of SMCR_EL1, the PE selects the Effective Streaming SVE vector length by performing checks in the following order: If the requested length is less than the minimum implemented Streaming SVE vector length, then the Effective length is the minimum implemented Streaming SVE vector length. If EL2 is implemented and enabled in the current Security state, and the requested length is greater than the Effective length at EL2, then the Effective length at EL2 is used. If EL3 is implemented and the requested length is greater than the Effective length at EL3, then the Effective length at EL3 is used. Otherwise, the Effective length is the highest supported Streaming SVE vector length that is less than or equal to the requested length. An indirect read of SMCR_EL1.LEN appears to occur in program order relative to a direct write of the same register, without the need for explicit synchronization. AU When the Effective value of HCR_EL2.E2H is 1, without explicit synchronization, accesses from EL3 using the accessor name SMCR_EL1 or SMCR_EL12 are not guaranteed to be ordered with respect to accesses using the other accessor name. MRS <Xt>, SMCR_EL1 if !IsFeatureImplemented(FEAT_SME) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif CPACR_EL1().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL1, 0x1D); elsif EL2Enabled() && !ELIsInHost(EL2) && CPTR_EL2().TSM == '1' then AArch64_SystemAccessTrap(EL2, 0x1D); elsif ELIsInHost(EL2) && CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; elsif EffectiveHCR_EL2_NVx() IN {'111'} then X{64}(t) = NVMem(0x1F0); else X{64}(t) = SMCR_EL1(); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif !ELIsInHost(EL2) && CPTR_EL2().TSM == '1' then AArch64_SystemAccessTrap(EL2, 0x1D); elsif ELIsInHost(EL2) && CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; elsif ELIsInHost(EL2) then X{64}(t) = SMCR_EL2(); else X{64}(t) = SMCR_EL1(); end; elsif PSTATE.EL == EL3 then if CPTR_EL3().ESM == '0' then AArch64_SystemAccessTrap(EL3, 0x1D); else X{64}(t) = SMCR_EL1(); end; end; MSR SMCR_EL1, <Xt> if !IsFeatureImplemented(FEAT_SME) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif CPACR_EL1().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL1, 0x1D); elsif EL2Enabled() && !ELIsInHost(EL2) && CPTR_EL2().TSM == '1' then AArch64_SystemAccessTrap(EL2, 0x1D); elsif ELIsInHost(EL2) && CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; elsif EffectiveHCR_EL2_NVx() IN {'111'} then NVMem(0x1F0) = X{64}(t); else SMCR_EL1() = X{64}(t); end; elsif PSTATE.EL == EL2 then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif !ELIsInHost(EL2) && CPTR_EL2().TSM == '1' then AArch64_SystemAccessTrap(EL2, 0x1D); elsif ELIsInHost(EL2) && CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; elsif ELIsInHost(EL2) then SMCR_EL2() = X{64}(t); else SMCR_EL1() = X{64}(t); end; elsif PSTATE.EL == EL3 then if CPTR_EL3().ESM == '0' then AArch64_SystemAccessTrap(EL3, 0x1D); else SMCR_EL1() = X{64}(t); end; end; MRS <Xt>, SMCR_EL12 When FEAT_VHE is implemented if !IsFeatureImplemented(FEAT_SME) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if EffectiveHCR_EL2_NVx() == '101' then X{64}(t) = NVMem(0x1F0); elsif EffectiveHCR_EL2_NVx() IN {'xx1'} then AArch64_SystemAccessTrap(EL2, 0x18); else Undefined(); end; elsif PSTATE.EL == EL2 then if ELIsInHost(EL2) then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; else X{64}(t) = SMCR_EL1(); end; else Undefined(); end; elsif PSTATE.EL == EL3 then if ELIsInHost(EL2) then if CPTR_EL3().ESM == '0' then AArch64_SystemAccessTrap(EL3, 0x1D); else X{64}(t) = SMCR_EL1(); end; else Undefined(); end; end; MSR SMCR_EL12, <Xt> When FEAT_VHE is implemented if !IsFeatureImplemented(FEAT_SME) then Undefined(); elsif PSTATE.EL == EL0 then Undefined(); elsif PSTATE.EL == EL1 then if EffectiveHCR_EL2_NVx() == '101' then NVMem(0x1F0) = X{64}(t); elsif EffectiveHCR_EL2_NVx() IN {'xx1'} then AArch64_SystemAccessTrap(EL2, 0x18); else Undefined(); end; elsif PSTATE.EL == EL2 then if ELIsInHost(EL2) then if HaveEL(EL3) && EL3SDDUndefPriority() && CPTR_EL3().ESM == '0' then Undefined(); elsif CPTR_EL2().SMEN IN {'x0'} then AArch64_SystemAccessTrap(EL2, 0x1D); elsif HaveEL(EL3) && CPTR_EL3().ESM == '0' then if EL3SDDUndef() then Undefined(); else AArch64_SystemAccessTrap(EL3, 0x1D); end; else SMCR_EL1() = X{64}(t); end; else Undefined(); end; elsif PSTATE.EL == EL3 then if ELIsInHost(EL2) then if CPTR_EL3().ESM == '0' then AArch64_SystemAccessTrap(EL3, 0x1D); else SMCR_EL1() = X{64}(t); end; else Undefined(); end; end; 2026-03-26 20:27:25 2026-03_rel