# EDECVIRTCHILD — Decrement VIRTCHILDCNT in SECS

Opcode/Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
EAX = 00H ENCLV[EDECVIRTCHILD] | IR | V/V | EAX[5] | This leaf function decrements the SECS VIRTCHILDCNT field.

## Instruction Operand Encoding ¶

Op/En | EAX | RBX | RCX
IR | EDECVIRTCHILD (In) | Return error code (Out) | Address of an enclave page (In) | Address of an SECS page (In)

### Description ¶

This instruction decrements the SECS VIRTCHILDCNT field. This instruction can only be executed when current privilege level is 0.

The content of RCX is an effective address of an EPC page. The DS segment is used to create linear address. Segment override is not supported.

## EDECVIRTCHILD Memory Parameter Semantics ¶

EPCPAGE | SECS
Read/Write access permitted by Non Enclave | Read access permitted by Enclave

The instruction faults if any of the following:

## EDECVIRTCHILD Faulting Conditions ¶

A memory operand effective address is outside the DS segment limit (32b mode). | A page fault occurs in accessing memory operands.
DS segment is unusable (32b mode). | RBX does not refer to an enclave page (REG, TCS, TRIM, SECS).
A memory address is in a non-canonical form (64b mode). | RCX does not refer to an SECS page.
A memory operand is not properly aligned. | RBX does not refer to an enclave page associated with SECS referenced in RCX.

### Concurrency Restrictions ¶

Leaf | Parameter | Base Concurrency Restrictions
--- | --- | ---
Access | On Conflict | SGX_CONFLICT VM Exit Qualification
EDECVIRTCHILD | Target [DS:RBX] | Shared | SGX_EPC_PAGE_ CONFLICT |
SECS [DS:RCX] | Concurrent |  |

Table 38-76. Base Concurrency Restrictions of EDECVIRTCHILD

Leaf | Parameter | Additional Concurrency Restrictions
--- | --- | ---
vs. EACCEPT, EACCEPTCOPY, EMODPE, EMODPR, EMODT | vs. EADD, EEXTEND, EINIT | vs. ETRACK, ETRACKC
Access | On Conflict | Access | On Conflict | Access | On Conflict
EDECVIRTCHILD | Target [DS:RBX] | Concurrent |  | Concurrent |  | Concurrent |
SECS [DS:RCX] | Concurrent |  | Concurrent |  | Concurrent |

Table 38-77. Additional Concurrency Restrictions of EDECVIRTCHILD

### Operation ¶

## Temp Variables in EDECVIRTCHILD Operational Flow ¶

Name | Type | Size (bits) | Description
--- | --- | --- | ---
TMP_SECS | Physical Address | 64 | Physical address of the SECS of the page being modified.
TMP_VIRTCHILDCNT | Integer | 64 | Number of virtual child pages.

## EDECVIRTCHILD Return Value in RAX ¶

Error | Value | Description
--- | --- | ---
No Error | 0 | EDECVIRTCHILD Successful.
SGX_EPC_PAGE_CONFLICT |  | Failure due to concurrent operation of another SGX instruction.
SGX_INVALID_COUNTER |  | Attempt to decrement counter that is already zero.

(* check alignment of DS:RBX *)

IF (DS:RBX is not 4K aligned) THEN

#GP(0); FI;

(* check DS:RBX is an linear address of an EPC page *)

IF (DS:RBX does not resolve within an EPC) THEN

#PF(DS:RBX, PFEC.SGX); FI;

(* check DS:RCX is an linear address of an EPC page *)

IF (DS:RCX does not resolve within an EPC) THEN

#PF(DS:RCX, PFEC.SGX); FI;

(* Check the EPCPAGE for concurrency *)

IF (EPCPAGE is being modified) THEN

RFLAGS.ZF = 1;

RAX = SGX_EPC_PAGE_CONFLICT;

goto DONE;

FI;

(* check that the EPC page is valid *)

IF (EPCM(DS:RBX).VALID = 0) THEN

#PF(DS:RBX, PFEC.SGX); FI;

(* check that the EPC page has the correct type and that the back pointer matches the pointer passed as the pointer to parent *)

IF ((EPCM(DS:RBX).PAGE_TYPE = PT_REG) or

(EPCM(DS:RBX).PAGE_TYPE = PT_TCS) or

(EPCM(DS:RBX).PAGE_TYPE = PT_TRIM) or

(EPCM(DS:RBX).PAGE_TYPE = PT_SS_FIRST) or

(EPCM(DS:RBX).PAGE_TYPE = PT_SS_REST))

THEN

(* get the SECS of DS:RBX *)

TMP_SECS := Address of SECS for (DS:RBX);

ELSE IF (EPCM(DS:RBX).PAGE_TYPE = PT_SECS) THEN

(* get the physical address of DS:RBX *)

TMP_SECS := Physical_Address(DS:RBX);

ELSE

(* EDECVIRTCHILD called on page of incorrect type *)

#PF(DS:RBX, PFEC.SGX); FI;

IF (TMP_SECS ≠ Physical_Address(DS:RCX)) THEN

#GP(0); FI;

(* Atomically decrement virtchild counter and check for underflow *)

Locked_Decrement(SECS(TMP_SECS).VIRTCHILDCNT);

IF (There was an underflow) THEN

Locked_Increment(SECS(TMP_SECS).VIRTCHILDCNT);

RFLAGS.ZF := 1;

RAX := SGX_INVALID_COUNTER;

goto DONE;

FI;

RFLAGS.ZF := 0;

RAX := 0;

DONE:

(* clear flags *)

RFLAGS.CF := 0;

RFLAGS.PF := 0;

RFLAGS.AF := 0;

RFLAGS.OF := 0;

RFLAGS.SF := 0;

### Flags Affected ¶

ZF is set if EDECVIRTCHILD fails due to concurrent operation with another SGX instruction, or if there is a VIRTCHILDCNT underflow. Otherwise cleared.

### Protected Mode Exceptions ¶

#GP(0) | If a memory operand effective address is outside the DS segment limit.
If DS segment is unusable.
If a memory operand is not properly aligned.
RBX does not refer to an enclave page associated with SECS referenced in RCX.
#PF(error | code) If a page fault occurs in accessing memory operands.
If RBX does not refer to an enclave page (REG, TCS, TRIM, SECS).
If RCX does not refer to an SECS page.

### 64-Bit Mode Exceptions ¶

#GP(0) | If a memory address is in a non-canonical form.
If a memory operand is not properly aligned.
RBX does not refer to an enclave page associated with SECS referenced in RCX.
#PF(error | code) If a page fault occurs in accessing memory operands.
If RBX does not refer to an enclave page (REG, TCS, TRIM, SECS).
If RCX does not refer to an SECS page.