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>

@@ -20,11 +20,11 @@
  */
 /*
  * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
  */
 /*
- * Copyright 2014 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright 2014-2015 Nexenta Systems, Inc.  All rights reserved.
  */
 
 #ifndef _IDM_IMPL_H_
 #define _IDM_IMPL_H_
 

@@ -407,13 +407,23 @@
 #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)) / sizeof (uint32_t))
+        ((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
  */