CFP RCTX

Control Flow Prediction Restriction by Context applies to all Control Flow Prediction Resources that predict execution based on information gathered within the target execution context or contexts.

Control flow predictions determined by the actions of code in the target execution context or contexts appearing in program order before the instruction cannot exploitatively control speculative execution occurring after the instruction is complete and synchronized.

This instruction is guaranteed to be complete following a DSB that covers both read and write behavior on the same PE as executed the original restriction instruction, and a subsequent context synchronization event is required to ensure that the effect of the completion of the instructions is synchronized to the current execution.

Note

This instruction does not require the invalidation of prediction structures so long as the behavior described for completion of this instruction is met by the implementation.

On some implementations the instruction is likely to take a significant number of cycles to execute. This instruction is expected to be used very rarely, such as on the roll-over of an ASID or VMID, but should not be used on every context switch.

Configuration

This instruction is present only when FEAT_SPECRES is implemented. Otherwise, direct accesses to CFP RCTX are UNDEFINED.

Attributes

Field descriptions

Bits [63:49]

GVMID, bit [48]

Execution of this instruction applies to all VMIDs or a specified VMID.

GVMID	Meaning
0b0	Applies to specified VMID for an EL0 or EL1 target execution context.
0b1	Applies to all VMIDs for an EL0 or EL1 target execution context.

For target execution contexts other than EL0 or EL1, this field is RES0.

If the instruction is executed at EL0 or EL1, this field has an Effective value of 0.

If EL2 is not implemented or not enabled for the target Security state, this field is RES0.

VMID, bits [47:32]

Only applies when bit[48] is 0 and the target execution context is either:

EL1.
EL0 when the Effective value of HCR_EL2.{E2H, TGE} is not {1, 1}.

Otherwise this field is RES0.

When the instruction is executed at EL1, this field is treated as the current VMID.

When the instruction is executed at EL0 and the Effective value of HCR_EL2.{E2H, TGE} is not {1, 1}, this field is treated as the current VMID.

When the instruction is executed at EL0 and the Effective value of HCR_EL2.{E2H, TGE} is {1, 1}, this field is ignored.

If EL2 is not implemented or not enabled for the target Security state, this field is RES0.

If the implementation supports 16 bits of VMID, then the upper 8 bits of the VMID must be written to 0 by software when the context being affected only uses 8 bits.

Bits [31:28]

NSE, bit [27]
When FEAT_RME is implemented:

Otherwise:

NS, bit [26]
When FEAT_RME is implemented:

Together with the NSE field, selects the Security state. Defined values are:

NSE	NS	Meaning
0b0	0b0	When Secure state is implemented, Secure. Otherwise reserved.
0b0	0b1	Non-secure.
0b1	0b0	Root.
0b1	0b1	Realm.

Some Effective values are determined by the current Security state:

When executed in Secure state, the Effective value of NSE is 0.
When executed in Non-secure state, the Effective value of {NSE, NS} is {0, 1}.
When executed in Realm state, the Effective value of {NSE, NS} is {1, 1}.

This instruction is treated as a NOP when executed at EL3 and either:

CFP_RCTX.{NSE, NS} selects a reserved value.
CFP_RCTX.{NSE, NS} == {1, 0} and CFP_RCTX.EL has a value other than 0b11.

Otherwise:

Security State. Defined values are:

NS	Meaning
0b0	Secure state.
0b1	Non-secure state.

When executed in Non-secure state, the Effective value of NS is 1.

EL, bits [25:24]

Exception Level. Indicates the Exception level of the target execution context.

EL	Meaning
0b00	EL0.
0b01	EL1.
0b10	EL2.
0b11	EL3.

If the instruction is executed at an Exception level lower than the specified level, or is specified to apply to a combination of Exception level and Security state that is not implemented, this instruction is treated as a NOP.

Bits [23:17]

GASID, bit [16]

Execution of this instruction applies to all ASIDs or a specified ASID.

GASID	Meaning
0b0	Applies to specified ASID for an EL0 target execution context.
0b1	Applies to all ASIDs for an EL0 target execution context.

For target execution contexts other than EL0, this field is RES0.

If the instruction is executed at EL0, this field has an Effective value of 0.

ASID, bits [15:0]

Only applies for an EL0 target execution context and when bit[16] is 0.

Otherwise, this field is RES0.

When the instruction is executed at EL0, this field is treated as the current ASID.

If the implementation supports 16 bits of ASID, then the upper 8 bits of the ASID must be written to 0 by software when the context being affected only uses 8 bits.

Executing CFP RCTX

Accesses to this instruction use the following encodings in the System instruction encoding space:

CFP RCTX, <Xt>

op0	op1	CRn	CRm	op2
0b01	0b011	0b0111	0b0011	0b100

if PSTATE.EL == EL0 then if !ELIsInHost(EL0) && SCTLR_EL1.EnRCTX == '0' then if EL2Enabled() && HCR_EL2.TGE == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.SystemAccessTrap(EL1, 0x18); elsif EL2Enabled() && !ELIsInHost(EL0) && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGITR_EL2.CFPRCTX == '1' then AArch64.SystemAccessTrap(EL2, 0x18); elsif ELIsInHost(EL0) && SCTLR_EL2.EnRCTX == '0' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.RestrictPrediction(X[t, 64], RestrictType_ControlFlow); elsif PSTATE.EL == EL1 then if EffectiveHCR_EL2_NVx() IN {'xx1'} then AArch64.SystemAccessTrap(EL2, 0x18); elsif EL2Enabled() && IsFeatureImplemented(FEAT_FGT) && (!HaveEL(EL3) || SCR_EL3.FGTEn == '1') && HFGITR_EL2.CFPRCTX == '1' then AArch64.SystemAccessTrap(EL2, 0x18); else AArch64.RestrictPrediction(X[t, 64], RestrictType_ControlFlow); elsif PSTATE.EL == EL2 then AArch64.RestrictPrediction(X[t, 64], RestrictType_ControlFlow); elsif PSTATE.EL == EL3 then AArch64.RestrictPrediction(X[t, 64], RestrictType_ControlFlow);

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
RES0															GVMID	VMID
RES0				NSE	NS	EL		RES0							GASID	ASID

CFP RCTX, Control Flow Prediction Restriction by Context

Purpose

Configuration

Attributes

Field descriptions

Bits [63:49]

GVMID, bit [48]

VMID, bits [47:32]

Bits [31:28]

NSE, bit [27]
When FEAT_RME is implemented:

Otherwise:

NS, bit [26]
When FEAT_RME is implemented:

Otherwise:

EL, bits [25:24]

Bits [23:17]

GASID, bit [16]

ASID, bits [15:0]

Executing CFP RCTX

CFP RCTX, <Xt>

CFP RCTX, Control Flow Prediction Restriction by Context

Purpose

Configuration

Attributes

Field descriptions

Bits [63:49]

GVMID, bit [48]

VMID, bits [47:32]

Bits [31:28]

NSE, bit [27]When FEAT_RME is implemented:

Otherwise:

NS, bit [26]When FEAT_RME is implemented:

Otherwise:

EL, bits [25:24]

Bits [23:17]

GASID, bit [16]

ASID, bits [15:0]

Executing CFP RCTX

CFP RCTX, <Xt>

NSE, bit [27]
When FEAT_RME is implemented:

NS, bit [26]
When FEAT_RME is implemented: