# EBLOCK — Mark a page in EPC as Blocked

Opcode/Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
EAX = 09H ENCLS[EBLOCK] | IR | V/V | SGX1 | This leaf function marks a page in the EPC as blocked.

## Instruction Operand Encoding ¶

Op/En | EAX | RCX
IR | EBLOCK (In) | Return error code (Out) | Effective address of the EPC page (In)

### Description ¶

This leaf function causes an EPC page to be marked as BLOCKED. 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.

An error code is returned in RAX.

The table below provides additional information on the memory parameter of EBLOCK leaf function.

## EBLOCK Memory Parameter Semantics ¶

EPCPAGE
Read/Write access permitted by Enclave

The error codes are:

Error Code (see Table 38-4) | Description
--- | ---
No Error | EBLOCK successful.
SGX_BLKSTATE | Page already blocked. This value is used to indicate to a VMM that the page was already in BLOCKED state as a result of EBLOCK and thus will need to be restored to this state when it is eventually reloaded (using ELDB).
SGX_ENTRYEPOCH_LOCKED | SECS locked for Entry Epoch update. This value indicates that an ETRACK is currently executing on the SECS. The EBLOCK should be reattempted.
SGX_NOTBLOCKABLE | Page type is not one which can be blocked.
SGX_PG_INVLD | Page is not valid and cannot be blocked.
SGX_EPC_PAGE_CONFLICT | Page is being written by EADD, EAUG, ECREATE, ELDU/B, EMODT, or EWB.

Table 38-12. EBLOCK Return Value in RAX

### Concurrency Restrictions ¶

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

Table 38-13. Base Concurrency Restrictions of EBLOCK

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

Table 38-14. Additional Concurrency Restrictions of EBLOCK

### Operation ¶

## Temp Variables in EBLOCK Operational Flow ¶

Name | Type | Size (Bits) | Description
TMP_BLKSTATE | Integer | 64 | Page is already blocked.

IF (DS:RCX is not 4KByte Aligned)

THEN #GP(0); FI;

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

THEN #PF(DS:RCX); FI;

RFLAGS.ZF,CF,PF,AF,OF,SF := 0;

RAX := 0;

(* Check the EPC page for concurrency*)

IF (EPC page in use)

THEN

RFLAGS.ZF := 1;

RAX := SGX_EPC_PAGE_CONFLICT;

GOTO DONE;

FI;

IF (EPCM(DS:RCX). VALID = 0)

THEN

RFLAGS.ZF := 1;

RAX := SGX_PG_INVLD;

GOTO DONE;

FI;

IF ( (EPCM(DS:RCX).PT ≠ PT_REG) and (EPCM(DS:RCX).PT ≠ PT_TCS) and (EPCM(DS:RCX).PT ≠ PT_TRIM)

and EPCM(DS:RCX).PT ≠ PT_SS_FIRST) and (EPCM(DS:RCX).PT ≠ PT_SS_REST))

THEN

RFLAGS.CF := 1;

IF (EPCM(DS:RCX).PT = PT_SECS)

THEN RAX := SGX_PG_IS_SECS;

ELSE RAX := SGX_NOTBLOCKABLE;

FI;

GOTO DONE;

FI;

(* Check if the page is already blocked and report blocked state *)

TMP_BLKSTATE := EPCM(DS:RCX).BLOCKED;

(* at this point, the page must be valid and PT_TCS or PT_REG or PT_TRIM*)

IF (TMP_BLKSTATE = 1)

THEN

RFLAGS.CF := 1;

RAX := SGX_BLKSTATE;

ELSE

EPCM(DS:RCX).BLOCKED := 1

FI;

DONE:

### Flags Affected ¶

Sets ZF if SECS is in use or invalid, otherwise cleared. Sets CF if page is BLOCKED or not blockable, otherwise cleared. Clears PF, AF, OF, SF.

### Protected Mode Exceptions ¶

#GP(0) | If a memory operand effective address is outside the DS segment limit.
If a memory operand is not properly aligned.
If the specified EPC resource is in use.
#PF(error | code) If a page fault occurs in accessing memory operands.
If a memory operand is not an EPC page.

### 64-Bit Mode Exceptions ¶

#GP(0) | If a memory operand is non-canonical form.
If a memory operand is not properly aligned.
If the specified EPC resource is in use.
#PF(error | code) If a page fault occurs in accessing memory operands.
If a memory operand is not an EPC page.