SCXTNUM

Provides a number that can be used to separate out different context numbers with the EL3 exception level, for the purpose of protecting against side-channels using branch prediction and similar resources.

Configuration

This register is present only when EL3 is implemented and (FEAT_CSV2_2 is implemented or FEAT_CSV2_1p2 is implemented). Otherwise, direct accesses to SCXTNUM_EL3 are UNDEFINED.

Attributes

Field descriptions

Bits [63:0]

Software Context Number. A number to identify the context within the EL3 exception level.

The reset behavior of this field is:

On a Warm reset, this field resets to an architecturally UNKNOWN value.

Accessing SCXTNUM_EL3

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

MRS <Xt>, SCXTNUM_EL3

op0	op1	CRn	CRm	op2
0b11	0b110	0b1101	0b0000	0b111

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then UNDEFINED; elsif PSTATE.EL == EL3 then X[t, 64] = SCXTNUM_EL3;

MSR SCXTNUM_EL3, <Xt>

op0	op1	CRn	CRm	op2
0b11	0b110	0b1101	0b0000	0b111

if PSTATE.EL == EL0 then UNDEFINED; elsif PSTATE.EL == EL1 then UNDEFINED; elsif PSTATE.EL == EL2 then UNDEFINED; elsif PSTATE.EL == EL3 then SCXTNUM_EL3 = X[t, 64];

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

SCXTNUM_EL3, EL3 Read/Write Software Context Number