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HSTR, Hyp System Trap Register

The HSTR characteristics are:

Purpose

Controls trapping to Hyp mode of Non-secure accesses, at EL1 or lower, to System registers in the coproc == 0b1111 encoding space:

  • By the CRn value used to access the register using MCR or MRC instruction.
  • By the CRm value used to access the register using MCRR or MRRC instruction.

Configuration

AArch32 System register HSTR bits [31:0] are architecturally mapped to AArch64 System register HSTR_EL2[31:0] .

If EL2 is not implemented, this register is RES0 from EL3.

Some or all RW fields of this register have defined reset values. These apply only if the PE resets into EL2 with EL2 using AArch32, or into EL3 with EL3 using AArch32. Otherwise, RW fields in this register reset to architecturally UNKNOWN values.

Attributes

HSTR is a 32-bit register.

Field descriptions

The HSTR bit assignments are:

313029282726252423222120191817161514131211109876543210
0000000000000000T15T14T13T12T11T10T9T8T7T6T5T4T3T2T1T0

Bits [31:16]

Reserved, RES0.

T<n>, bit [n], for n = 0 to 15

Fields T14 and T4 are RES0.

The remaining fields control whether Non-secure EL0 and EL1 accesses, using MCR, MRC, MCRR, and MRRC instructions, to the System registers in the coproc == 0b1111 encoding space are trapped to Hyp mode:

T<n>Meaning
0b0

This control has no effect on Non-secure EL0 or EL1 accesses to System registers.

0b1

Any Non-secure EL1 MCR or MRC access with coproc == 0b1111 and CRn == <n> is trapped to Hyp mode. A Non-secure EL0 MCR or MRC access with these values is trapped to Hyp mode only if the access is not UNDEFINED when the value of this field is 0.

Any Non-secure EL1 MCRR or MRRC access with coproc == 0b1111 and CRm == <n> is trapped to Hyp mode. A Non-secure EL0 MCRR or MRRC access with these values is trapped to Hyp mode only if the access is not UNDEFINED when the value of this field is 0.

For example, when HSTR.T7 is 1, for instructions executed at Non-secure EL1:

  • An MCR or MRC instruction with coproc set to 0b1111 and <CRn> set to c7 is trapped to Hyp mode.
  • An MCRR or MRRC instruction with coproc set to 0b1111 and <CRm> set to c7 is trapped to Hyp mode.

In a system where the PE resets into EL2 or EL3, this field resets to 0.

Accessing the HSTR

Accesses to this register use the following encodings:

MRC{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <CRn>, <CRm>{, {#}<opc2>}

opc1opc2CRncoprocCRm
0b1000b0110b00010b11110b0001
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T1 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T1 == '1' then
        AArch32.TakeHypTrapException(0x03);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    return HSTR;
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' then
        UNDEFINED;
    else
        return HSTR;
              

MCR{<c>}{<q>} <coproc>, {#}<opc1>, <Rt>, <CRn>, <CRm>{, {#}<opc2>}

opc1opc2CRncoprocCRm
0b1000b0110b00010b11110b0001
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && !ELUsingAArch32(EL2) && HSTR_EL2.T1 == '1' then
        AArch64.AArch32SystemAccessTrap(EL2, 0x03);
    elsif EL2Enabled() && ELUsingAArch32(EL2) && HSTR.T1 == '1' then
        AArch32.TakeHypTrapException(0x03);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    HSTR = R[t];
elsif PSTATE.EL == EL3 then
    if SCR.NS == '0' then
        UNDEFINED;
    else
        HSTR = R[t];
              


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