HP OpenVMS systems documentation

The Direct I/O Buffer Map (DIOBM) is a new structure that is used to solve the "cross-process PTE problem" for buffers that have been locked into memory for direct I/O.

There are two variants of the DIOBM structure. The first is the primary DIOBM structure. The primary DIOBM structure can be used in the following mutually exclusive ways:

1. To contain copies of the actual PTEs that map the buffer.
2. To point to a larger secondary DIOBM structure if the primary DIOBM structure has insufficient room for all the PTEs that map the user buffer.
3. To manage a PTE window in S0/S1 space onto the actual PTEs that map the buffer if the required PTE count exceeds the capacity of the largest allowable DIOBM structure.

Each of these methods yields a 32-bit system virtual address for the PTEs that map the buffer. This address is valid regardless of process or system context.

The fixed-size DIOBM structure contains room for exactly DIOBM$K_PTECNT_FIX (9) PTEs and is 88 bytes long. Most primary DIOBM structures are fixed-sized and embedded in other structures. For example, the IRP, IRPE, VCRP, and DCBE structures all contain an embedded fixed-sized primary DIOBM structure.

A secondary DIOBM structure can have room for up to ioc$gl_diobm_ptecnt_max PTEs and is used only for PTE copies.

Although the offsets and types for both the primary and secondary DIOBM structures are identical, for clarity, they are described in separate tables (see Table A-6 and Table A-7).

Table A-6 Primary DIOBM Structure

Field	Type	Comments
diobm$ps_aux_diobm	DIOBM *	This is a pointer to a secondary DIOBM structure that is valid if and only if DIOBM$M_AUX_INUSE in diobm$l_flags is set. The secondary DIOBM structure contains copies of the PTE that map the buffer. When a secondary DIOBM is used, the only use for the primary DIOBM is to locate the secondary.
diobm$l_pte_count	unsigned int	If DIOBM$M_PTE_WINDOW is clear in diobm$l_flags , this cell contains the count of PTEs that have been copied to the PTE vector diobm$q_pte_vector in this DIOBM structure. If DIOBM$M_PTE_WINDOW is set in diobm$l_flags , this cell contains the count of SPTEs that have been allocated for a PTE window in S0/S1 space to the actual PTEs that map the buffer.
diobm$w_size	unsigned short	Size of the DIOBM packet in bytes.
diobm$b_type	unsigned char	Nonpaged pool packet type code, DYN$C_MISC
diobm$b_subtype	unsigned char	Nonpaged pool packet subtype code, new DYN$C_MISC subtype code DYN$C_DIOBM
diobm$l_flags	unsigned int	Flag bits.
diobm$v_rel_dealloc	bit	A bit equal to DIOBM$M_REL_DEALLOC in the diobm$l_flags cell. If set, routine IOC_STD$RELEASE_DIOBM deallocates this DIOBM structure. The routine IOC_STD$FILL_DIOBM sets this bit on any secondary DIOBM structure that it may allocate. The routine IOC_STD$CREATE_DIOBM sets this bit on the primary DIOBM structure that it allocates.
diobm$v_pte_window	bit	A bit equal to DIOBM$M_PTE_WINDOW in the diobm$l_flags cell. This bit is set if the direct I/O buffer is too large for a DIOBM packet (the buffer requires more than ioc$gl_diobm_ptecnt_max PTEs) and a window in S0 to its PTEs has been allocated. When this bit is set, diobm$l_pte_count contains the count of SPTEs that have been allocated and the diobm$l_ptew_sva cell contains the system virtual address that is mapped by the first SPTE allocated for the PTE window. This bit must be clear if diobm$v_aux_inuse is set.
diobm$v_aux_inuse	bit	A bit equal to DIOBM$M_AUX_INUSE in the diobm$l_flags cell. The diobm$ps_aux_diobm cell contains a pointer to a secondary DIOBM structure if and only if the diobm$v_aux_inuse bit is set. This bit must be clear if diobm$v_pte_window is set.
diobm$v_inuse	bit	A bit equal to DIOBM$M_INUSE in the diobm$l_flags cell. This flag is an aid to detecting inproper use of DIOBM structures and is used only by the full-checking versions of the routines in the IO_ROUTINES_MON.EXE execlet. This flag is set by the IOC_STD$FILL_DIOBM and IOC_STD$CREATE_DIOBM routines and is cleared by the IOC_STD$RELEASE_DIOBM routine. Prior to setting the flag, the IOC_STD$FILL_DIOBM routine checks this flag if the diobm$b_type cell contains the DYN$C_MISC value and diobm$b_subtype contains DYN$C_DIOBM. If the diobm$v_inuse flag is set under these conditions, the IOC_STD$FILL_DIOBM routine declares a INCONSTATE bugcheck.
diobm$v_s0pte_window	bit	A bit equal to DIOBM$M_S0PTE_WINDOW in the diobm$l_flags cell. This bit is set if the S0/S1 PTE window was used to derive a 32-bit PTE address for this buffer. When this bit is set the diobm$v_pte_window and diobm$v_aux_inuse flags must be clear and the diobm$l_pte_count cell must contain 0.
DIOBM$K_HDRLEN	constant	Size in bytes of the minimal DIOBM packet header excluding the PTE vector. This is equal to the byte offset of the diobm$q_pte_vector[0] cell (16).
diobm$q_pte_vector	PTE[diobm$l_pte_count]	Vector of diobm$l_pte_count quadword PTEs that are copies of the PTEs that map the buffer that has been locked for direct I/O. This vector is valid only if both DIOBM$M_AUX_INUSE and DIOBM$M_PTE_WINDOW in diobm$l_flags are clear.
DIOBM$K_PTECNT_FIX	constant	This constant specifies the number of PTE entries (9) that fit into the PTE vector in a fix-sized DIOBM structure.
DIOBM$K_PTECNT_MAX_UNI	constant	This constant specifies the number of PTE entries (94) that fit into the PTE vector in the largest allowable DIOBM structure on an uniprocessor system.
DIOBM$K_PTECNT_MAX_SMP	constant	This constant specifies the number of PTE entries (430) that fit into the PTE vector in the largest allowable DIOBM structure on an SMP system.
diobm$ps_ptew_sva	void *	The lowest S0/S1 space virtual address that is mapped by the PTEs that have been allocated for the window onto the direct I/O buffer PTEs. This cell is used to deallocate the PTE window. This cell is overlaid on a portion of diobm$q_pte_vector since its use is mutually exclusive. This cell is valid if and only if DIOBM$M_PTE_WINDOW in diobm$l_flags is set.
DIOBM$M_NORESWAIT	constant	This is an option bit mask for the flags parameter to the IOC_STD$FILL_DIOBM and IOC_STD$CREATE_DIOBM routines. When this option bit is set and there are insufficient resources for the needs of these routines an error status is returned to their callers instead of putting the process into a resource wait state.

Table A-7 Secondary DIOBM Structure

Field	Type	Comments
diobm$ps_aux_diobm	DIOBM *	This cell must be zero in a secondary DIOBM structure.
diobm$l_pte_count	unsigned int	Contains the number of PTEs that can fit into the diobm$q_pte_vector in this DIOBM structure.
diobm$w_size	unsigned short	Size of the DIOBM packet in bytes.
diobm$b_type	unsigned char	Nonpaged pool packet type code, DYN$C_MISC
diobm$b_subtype	unsigned char	Nonpaged pool packet subtype code, new DYN$C_MISC subtype code DYN$C_DIOBM
diobm$l_flags	unsigned int	Flag bits.
diobm$v_rel_dealloc	bit	A bit equal to DIOBM$M_REL_DEALLOC in the diobm$l_flags cell. If set, routine IOC_STD$RELEASE_DIOBM deallocates this DIOBM structure.
diobm$v_pte_window	bit	A bit equal to DIOBM$M_PTE_WINDOW in the diobm$l_flags cell. This bit must be clear in a secondary DIOBM structure.
diobm$v_aux_inuse	bit	A bit equal to DIOBM$M_AUX_INUSE in the diobm$l_flags cell. This bit must be clear in a secondary DIOBM structure.
diobm$v_s0pte_window	bit	A bit equal to DIOBM$M_S0PTE_WINDOW in the diobm$l_flags cell. This bit must be clear in a secondary DIOBM structure.
diobm$q_pte_vector	PTE[diobm$l_pte_count]	Vector of diobm$l_pte_count quadword PTEs that are copies of the PTEs that map the buffer that has been locked for direct I/O.