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SCXTNUM_EL3, EL3 Read/Write Software Context Number

The SCXTNUM_EL3 characteristics are:

Purpose

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 ARMv8.0-CSV2 is implemented. Otherwise, direct accesses to SCXTNUM_EL3 are UNDEFINED.

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

SCXTNUM_EL3 is a 64-bit register.

Field descriptions

The SCXTNUM_EL3 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
Software Context Number
Software Context Number
313029282726252423222120191817161514131211109876543210

Bits [63:0]

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

This field resets to an architecturally UNKNOWN value.

Accessing the SCXTNUM_EL3

Accesses to this register use the following encodings:

MRS <Xt>, SCXTNUM_EL3

op0CRnop1op2CRm
0b110b11010b1100b1110b0000
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    UNDEFINED;
elsif PSTATE.EL == EL2 then
    UNDEFINED;
elsif PSTATE.EL == EL3 then
    return SCXTNUM_EL3;
              

MSR SCXTNUM_EL3, <Xt>

op0CRnop1op2CRm
0b110b11010b1100b1110b0000
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];
              


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