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HSTR_EL2, Hypervisor System Trap Register

The HSTR_EL2 characteristics are:

Purpose

Controls trapping to EL2 of EL1 or lower AArch32 accesses to the System register in the coproc == 0b1111 encoding space, by the CRn value used to access the register using MCR or MRC instruction. When the register is accessible using an MCRR or MRRC instruction, this is the CRm value used to access the register.

Configuration

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

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

If no Exception level can use AArch32, then this register is RES0.

This register has no effect if EL2 is not enabled in the current Security state.

RW fields in this register reset to architecturally UNKNOWN values.

Attributes

HSTR_EL2 is a 64-bit register.

Field descriptions

The HSTR_EL2 bit assignments are:

6362616059585756555453525150494847464544434241403938373635343332
00000000000000000000000000000000
0000000000000000T15T14T13T12T11T10T9T8T7T6T5T4T3T2T1T0
313029282726252423222120191817161514131211109876543210

Bits [63:16]

Reserved, RES0.

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

Fields T14 and T4 are RES0.

The remaining fields control whether EL0 and EL1 accesses, using MCR, MRC, MCRR, and MRRC instructions, to the System registers in the coproc == 0b1111 encoding space are trapped to EL2:

T<n>Meaning
0b0

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

0b1

Any EL1 MCR or MRC access with coproc == 0b1111 and CRn == <n> is trapped to EL2. An EL0 MCR or MRC access with these values is trapped to EL2 only if the access is not UNDEFINED when the value of this field is 0.

Any EL1 MCRR or MRRC access with coproc == 0b1111 and CRm == <n> is trapped to EL2. An EL0 MCRR or MRRC access with these values is trapped to EL2 only if the access is not UNDEFINED when the value of this field is 0.

For example, when HSTR_EL2.T7 is 1, for instructions executed at EL1:

  • An MCR or MRC instruction with coproc set to 0b1111 and <CRn> set to c7 is trapped to EL2.
  • An MCRR or MRRC instruction with coproc set to 0b1111 and <CRm> set to c7 is trapped to EL2.

When ARMv8.1-VHE is implemented, and the value of HCR_EL2.{E2H, TGE} is {1, 1}, this field behaves as 0 for all purposes other than a direct read of the value of this bit.

This field resets to an architecturally UNKNOWN value.

Accessing the HSTR_EL2

Accesses to this register use the following encodings:

MRS <Xt>, HSTR_EL2

op0CRnop1op2CRm
0b110b00010b1000b0110b0001
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.<NV2,NV> == '11' then
        return NVMem[0x080];
    elsif EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    return HSTR_EL2;
elsif PSTATE.EL == EL3 then
    return HSTR_EL2;
              

MSR HSTR_EL2, <Xt>

op0CRnop1op2CRm
0b110b00010b1000b0110b0001
if PSTATE.EL == EL0 then
    UNDEFINED;
elsif PSTATE.EL == EL1 then
    if EL2Enabled() && HCR_EL2.<NV2,NV> == '11' then
        NVMem[0x080] = X[t];
    elsif EL2Enabled() && HCR_EL2.NV == '1' then
        AArch64.SystemAccessTrap(EL2, 0x18);
    else
        UNDEFINED;
elsif PSTATE.EL == EL2 then
    HSTR_EL2 = X[t];
elsif PSTATE.EL == EL3 then
    HSTR_EL2 = X[t];
              


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