NEX-3672 IDM module panics target when PDU has AHS length between 17 and 49
Reviewed by: Steve Peng <steve.peng@nexenta.com>
Reviewed by: Rob Gittins <rob.gittins@nexenta.com>

*** 20,30 ****
   */
  /*
   * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
   */
  /*
!  * Copyright 2014 Nexenta Systems, Inc.  All rights reserved.
   */
  
  #ifndef _IDM_IMPL_H_
  #define _IDM_IMPL_H_
  
--- 20,30 ----
   */
  /*
   * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
   */
  /*
!  * Copyright 2014-2015 Nexenta Systems, Inc.  All rights reserved.
   */
  
  #ifndef _IDM_IMPL_H_
  #define _IDM_IMPL_H_
  
*** 407,419 ****
  #define IDM_PDU_SET_STATSN      0x00000010
  #define IDM_PDU_ADVANCE_STATSN  0x00000020
  
  #define OSD_EXT_CDB_AHSLEN      (200 - 15)
  #define BIDI_AHS_LENGTH         5
  #define IDM_SORX_CACHE_AHSLEN \
!         (((OSD_EXT_CDB_AHSLEN + 3) + \
!             (BIDI_AHS_LENGTH + 3)) / sizeof (uint32_t))
  #define IDM_SORX_CACHE_HDRLEN   (sizeof (iscsi_hdr_t) + IDM_SORX_CACHE_AHSLEN)
  
  /*
   * ID pool
   */
--- 407,429 ----
  #define IDM_PDU_SET_STATSN      0x00000010
  #define IDM_PDU_ADVANCE_STATSN  0x00000020
  
  #define OSD_EXT_CDB_AHSLEN      (200 - 15)
  #define BIDI_AHS_LENGTH         5
+ /*
+  * Additional Header Segment (AHS)
+  * AHS is only valid for SCSI Requests and contains SCSI CDB information
+  * which doesn't fit in the standard 16 byte area of the PDU. Commonly
+  * this only holds true for OSD device commands.
+  *
+  * IDM_SORX_CACHE_ASHLEN is the amount of memory which is preallocated in bytes.
+  * When used in the header the AHS length is stored as the number of 4-byte
+  * words; so IDM_SORX_WIRE_ASHLEN is IDM_SORX_CACHE_ASHLEN in words.
+  */
  #define IDM_SORX_CACHE_AHSLEN \
!         ((OSD_EXT_CDB_AHSLEN + 3) + (BIDI_AHS_LENGTH + 3))
! #define IDM_SORX_WIRE_AHSLEN (IDM_SORX_CACHE_AHSLEN / sizeof (uint32_t))
  #define IDM_SORX_CACHE_HDRLEN   (sizeof (iscsi_hdr_t) + IDM_SORX_CACHE_AHSLEN)
  
  /*
   * ID pool
   */