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

SCXTNUM, 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 !(HaveEL(EL3) && (IsFeatureImplemented(FEAT_CSV2_2) || IsFeatureImplemented(FEAT_CSV2_1p2))) then UNDEFINED; elsif 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

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
SCXTNUM
SCXTNUM

SCXTNUM_EL3, EL3 Read/Write Software Context Number