scst/doc/scst_user_spec.txt

	SCST USER SPACE DEVICE HANDLER MODULE.

	  USER SPACE INTERFACE DESCRIPTION.

		   Version 1.0.0


		I. Description.

SCST user space device handler module scst_user is a device handler for
SCST, which provides a way to implement in the user space complete, full
feature virtual SCSI devices in the SCST environment.

This document assumes that the reader is familiar with the SCST
architecture and the states through which SCSI commands go during
processing in SCST. Module scst_user basically only provides hooks to
them. Their description could be found on the SCST web page on
http://scst.st.net.


		II. User space API.

Module scst_user provides /dev/scst_user character device with the
following system calls available: 

 - open() - allows to open the device and get a file handle, which
   will be used in all subsequent actions until close() is called

 - close() - closes file handle returned by open()
 
 - poll() - allows to wait until some pending command from SCST to
   process is available.

 - ioctl() - main call, which allows commands interchange with the SCST
   core.

Device /dev/scst_user could be opened in blocking or non-blocking mode
using O_NONBLOCK flag. In the blocking mode ioctl()
SCST_USER_REPLY_GET_CMD function blocks until there is a new subcommand
to process. In the non-blocking mode if there are no pending subcommands
SCST_USER_REPLY_GET_CMD function returns immediately with EAGAIN error
code, and the user space device handler can use poll() call to get
notification about new subcommands arrival. The blocking mode is the
default.

The module scst_user API is defined in scst_user.h file.


		III. IOCTL() functions.

There are following IOCTL functions available. All of them has one
argument. They all, except SCST_USER_REGISTER_DEVICE return 0 for
success or -1 in case of error, and errno is set appropriately.


		1. SCST_USER_REGISTER_DEVICE

SCST_USER_REGISTER_DEVICE - registers new virtual user space device. The
argument is:

struct scst_user_dev_desc
{
	uint8_t version;
	uint8_t type;
	uint8_t has_own_order_mgmt;
	struct scst_user_opt opt;
	uint32_t block_size;
	char name[SCST_MAX_NAME];
},

where:

 - version - is a protocol version, shall be DEV_USER_VERSION

 - type - SCSI type of the device

 - has_own_order_mgmt - set it in non-zero, if device implements own
   ORDERED commands management, i.e. guarantees commands execution order
   requirements, specified by SAM.

 - opt - device options, see SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS below

 - block_size - block size, shall be divisible by 512 for block devices

 - name - name of the device

SCST_USER_REGISTER_DEVICE returns registered device's handler or -1
in case of error, and errno is set appropriately.

In order to unregister the device, either call SCST_USER_UNREGISTER_DEVICE
function, or close on its file descriptor.


		2. SCST_USER_UNREGISTER_DEVICE

SCST_USER_UNREGISTER_DEVICE - unregisters the corresponding virtual user
space device. It doesn't have any parameters.

During execution of SCST_USER_UNREGISTER_DEVICE at least one another
thread must process all coming subcommands, otherwise after timeout it
will fail with EBUSY error.

SCST_USER_UNREGISTER_DEVICE returns 0 on success or -1 in case of error,
and errno is set appropriately.


		3. SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS

SCST_USER_SET_OPTIONS/SCST_USER_GET_OPTIONS allows to set or get
correspondingly various options that control various aspects of SCSI
commands processing.

The argument is:

struct scst_user_opt
{
	uint8_t parse_type;
	uint8_t on_free_cmd_type;
	uint8_t memory_reuse_type;
	uint8_t partial_transfers_type;
	uint32_t partial_len;

	uint8_t tst;
	uint8_t queue_alg;
	uint8_t tas;
	uint8_t swp;
},

where:

 - parse_type - defines how the user space handler wants to process
   PARSE subcommand. Possible values are:
  
   * SCST_USER_PARSE_STANDARD - tells SCST use standard internal parser
     for this SCSI device type.
   
   * SCST_USER_PARSE_CALL - tells SCST generate SCST_USER_PARSE for all
     SCSI commands

   * SCST_USER_PARSE_EXCEPTION - tells SCST generate SCST_USER_PARSE
     for unknown SCSI commands or SCSI commands that produce errors in
     the standard parser.

 - on_free_cmd_type - defines how the user space handler wants to
   process ON_FREE_CMD subcommand. Possible values are:

   * SCST_USER_ON_FREE_CMD_CALL - tells SCST generate
     SCST_USER_ON_FREE_CMD for all SCSI commands

   * SCST_USER_ON_FREE_CMD_IGNORE - tells SCST do nothing on this event.

 - memory_reuse_type - defines how memory allocated by the user space
   handler for a SCSI commands data buffers is then reused by the SCST
   core as data buffer for subsequent commands. Possible values are:
   
   * SCST_USER_MEM_NO_REUSE - no memory reuse is possible, for each
     commands the user space handler will each time allocate a dedicated
     data buffer

   * SCST_USER_MEM_REUSE_READ - memory reuse by only READ-type commands
     (i.e. which involve data transfer from target to initiator) is
     allowed. For all WRITE-type commands (i.e. which involves data
     transfer from initiator to target) the user space handler will
     each time allocate a dedicated data buffer

   * SCST_USER_MEM_REUSE_WRITE - memory reuse by only WRITE-type
     commands is allowed. For all WRITE-type commands the user space
     handler will each time allocate a dedicated data buffer

   * SCST_USER_MEM_REUSE_ALL - unlimited memory reuse is possible.

 - partial_transfers_type - defines if the user space handler supports
   partial data transfers, when a SCSI command, which required big data
   transfer, is broken on several subcommands with smaller data
   transfers. This allows to improve performance by simultaneous data
   transfers from/to remote initiator and to/from the underlying storage
   device as well as lower allocation memory requirements for each
   (sub)command. All subcommands will have the same unique value in
   "parent_cmd_h" field and SCST_USER_SUBCOMMAND flag in "partial" field
   of struct scst_user_scsi_cmd_exec. The final subcommand will also
   have in that field SCST_USER_SUBCOMMAND_FINAL flag. All the
   subcommands will have the original unmodified CDB. Possible values
   are:
   
   * SCST_USER_PARTIAL_TRANSFERS_NOT_SUPPORTED - the partial data
     transfers are not supported

   * SCST_USER_PARTIAL_TRANSFERS_SUPPORTED_ORDERED - the partial data
     transfers are supported, but all the subcommands must come in order
     of data chunks. Could be used, e.g., for tape devices.

   * SCST_USER_PARTIAL_TRANSFERS_SUPPORTED - the partial data transfers
     are supported without limitations.

 - tst, queue_alg, tas and swp - set values for TST, QUEUE ALGORITHM MODIFIER,
   TAS and SWP fields from control mode page correspondingly, see SPC.

Flags parse_type and on_free_cmd_type are designed to improve
performance by eliminating context switches to the user space handler,
when processing of the corresponding events isn't needed.

Flag memory_reuse_type is designed to improve performance by eliminating
memory allocation, preparation and then freeing each time for each
commands, if the same memory will be allocated again and again. See
SCST_USER_ALLOC_MEM description below for more info.

SCST_USER_SET_OPTIONS should not be used from the same and the only
thread, which also handles incoming commands, otherwise there could be a
"deadlock", when SCST_USER_SET_OPTIONS waits for active commands finish,
but nobody handles them. This "deadlock" will be resolved only when
initiator, which sent those commands, aborts them after timeout.


		4. SCST_USER_REPLY_AND_GET_CMD

SCST_USER_REPLY_AND_GET_CMD allows at one call reply on the current
subcommand from SCST and get the next one. If 0 is returned by ioctl(),
SCST_USER_REPLY_AND_GET_CMD returns a SCST subcommand in the argument,
which is defined as the following:

struct scst_user_get_cmd
{
	uint32_t cmd_h;
	uint32_t subcode;
	union {
		uint64_t preply;
		struct scst_user_sess sess;
		struct scst_user_scsi_cmd_parse parse_cmd;
		struct scst_user_scsi_cmd_alloc_mem alloc_cmd;
		struct scst_user_scsi_cmd_exec exec_cmd;
		struct scst_user_scsi_on_free_cmd on_free_cmd;
		struct scst_user_on_cached_mem_free on_cached_mem_free;
		struct scst_user_tm tm_cmd;
	};
},

where:

 - cmd_h - command handle used to identify the command in the reply.

 - subcode - subcommand code, see 4.1 below

  - preply - pointer to the reply data or, if 0, there is no reply. See
   SCST_USER_REPLY_CMD for description of struct scst_user_reply_cmd
   fields

Other union members contain command's specific payload.

For all received subcommands the user space device handler shall call
SCST_USER_REPLY_AND_GET_CMD or SCST_USER_REPLY_CMD function to tell SCST
that the subcommand's processing is finished, although some subcommands
don't return a value.


		4.1. Possible subcommands:


		4.1.1. SCST_USER_ATTACH_SESS

SCST_USER_ATTACH_SESS notifies the user space handler that a new
initiator's session is about to be attached to the device. Payload
contains struct scst_user_sess, which is defined as the following:

struct scst_user_sess
{
	uint64_t sess_h;
	uint64_t lun;
	uint16_t threads_num;
	uint8_t rd_only;
	char initiator_name[SCST_MAX_NAME];
	char target_name[SCST_MAX_NAME];
},

where:

 - sess_h - session's handle, may not be 0

 - lun - assigned LUN for this device in this session

 - threads_num - specifies amount of additional threads, requested by
   the corresponding target driver

 - rd_only - if true, this device is read only in this session

 - initiator_name - name of the remote initiator, which initiated this
   session

 - target_name - name of the target, to which this session belongs

When SCST_USER_ATTACH_SESS is returned, it is guaranteed that there are
no other commands are being executed or pending.
 
After SCST_USER_ATTACH_SESS function completed, the user space device
handler shall reply using "result" field of the corresponding reply
command.


		4.1.2. SCST_USER_DETACH_SESS

SCST_USER_DETACH_SESS notifies the user space handler that the
corresponding initiator is about to be detached from the particular
device. Payload contains struct scst_user_sess, where only handle field
is valid.

When SCST_USER_DETACH_SESS is returned, it is guaranteed that there are
no other commands are being executed or pending.

This command doesn't reply any return value, although
SCST_USER_REPLY_AND_GET_CMD or SCST_USER_REPLY_CMD function must be
called.


		4.1.3. SCST_USER_PARSE

SCST_USER_PARSE returns SCSI command on PARSE state of the SCST
processing. The PARSE state is intended to check validity of the
command, determine data transfer type and the necessary data buffer
size. This subcommand is returned only if SCST_USER_SET_OPTIONS
parse_type isn't set to SCST_USER_PARSE_STANDARD. In this case the
standard SCST internal parser for this SCSI device type will do all the
job.

Payload contains struct scst_user_scsi_cmd_parse, which is defined as
the following:

struct scst_user_scsi_cmd_parse
{
	uint64_t sess_h;

	uint8_t cdb[SCST_MAX_CDB_SIZE];
	int32_t cdb_len;

	uint32_t timeout;
	int32_t bufflen;

	uint8_t queue_type;
	uint8_t data_direction;

	uint8_t expected_values_set;
	uint8_t expected_data_direction;
	int32_t expected_transfer_len;

	uint32_t sn;
},

where:

 - sess_h - corresponding session handler

 - cdb - SCSI CDB

 - cdb_len - SCSI CDB length
 
 - timeout - CDB execution timeout

 - bufflen - command's buffer length
 
 - queue_type - SCSI task attribute (queue type)
 
 - data_direction - command's data flow direction, one of SCST_DATA_*
   constants

 - expected_values_set - true if expected_data_direction and
   expected_transfer_len contain valid values
   
 - expected_data_direction - remote initiator supplied command's data
   flow direction

 - expected_transfer_len - remote initiator supplied transfer length

 - sn - command's SN, which might be used for task management

In the PARSE state of SCSI commands processing the user space device
handler shall check and provide SCST values for command data buffer
length, data flow direction and timeout, which it shall reply using the
corresponding reply command.

In case of any error the error reporting should be deferred until
SCST_USER_EXEC subcommand, where the appropriate SAM status and sense
shall be set.
   

		4.4.4. SCST_USER_ALLOC_MEM

SCST_USER_ALLOC_MEM returns SCSI command on memory allocation state of
the SCST processing. On this state the user space device handler shall
allocate the command's data buffer with bufflen length and then return
it to SCST using the corresponding reply command. Then SCST internally
will convert it in SG vector in order to use it itself and by target
drivers.

If the memory reuse type is disabled (i.e. set to SCST_USER_MEM_NO_REUSE)
there are no special requirements for buffer memory or its alignment, it
could be just what malloc() returned. If the memory reuse type is enabled,
the buffer shall be page size aligned, for example using memalign()
function.

Payload contains struct scst_user_scsi_cmd_alloc_mem, which is defined
as the following:

struct scst_user_scsi_cmd_alloc_mem
{
	uint64_t sess_h;

	uint8_t cdb[SCST_MAX_CDB_SIZE];
	int32_t cdb_len;

	int32_t alloc_len;

	uint8_t queue_type;
	uint8_t data_direction;

	uint32_t sn;
},

where:

 - sess_h - corresponding session handler

 - cdb - SCSI CDB

 - cdb_len - SCSI CDB length

 - alloc_len - command's buffer length
 
 - queue_type - SCSI task attribute (queue type )

 - data_direction - command's data flow direction, one of SCST_DATA_*
   constants

 - sn - command's SN, which might be used for task management

Memory allocation, preparation and freeing are ones of the most
complicated and expensive operations during SCSI commands processing.
Module scst_user provides a way to almost completely eliminate those
operations by reusing once allocated memory for subsequent SCSI
commands. It is controlled by memory_reuse_type option, which could be
set by SCST_USER_SET_OPTIONS function. If any type memory reusage is
enabled, then SCST will use its internal SGV cache in order to cache
allocated and fully built SG vectors for subsequent commands of this
type, so for them SCST_USER_ALLOC_MEM subfunction will not be called and
in SCST_USER_EXEC pbuf pointer will point to that reused buffer. 

SGV cache is a backend cache made on top of Linux kernel SLAB cache. It
caches SG vectors with power of 2 data sizes: 4K, 8K, ..., 4M. So, if
there is a 5K SCSI command coming, then the user space handler will be
asked for 8K allocation, from which only 5K will be used for this
particular SCSI command. Then that SG vector will be cached and
subsequently reused for all SCSI commands between 4K and 8K. For a 1M
SCSI command the user space handler will be asked for another buffer
(while the previous 5K one will be kept), which will not be then reused
for 5K SCSI commands, since 1M and 5K vectors belong to different cache
data sizes, it will be reused only for commands between 512K and 1M.
Then, after some time of inactivity or when the system is under memory
pressure, the cache entries will be freed and the user space handler
will be notified using SCST_USER_ON_CACHED_MEM_FREE.

Since SGV cache caches SG vectors with power of 2 data sizes, alloc_len
field could be up to 2 times more, than actually required by the SCSI
command.

The memory reuse could be used in both SCSI tagged and untagged queuing
environments. In the SCSI tagged queuing environment the SGV cache will
take care that several commands don't use the same buffer simultaneously
by asking the user space handler to allocate a new data buffer, when all
cached ones are busy.

Some important notes:

1. If the user space handler needs to call fork(), it must call
madvise() with MADV_DONTFORK flag for all allocated data buffers,
otherwise parent or child process could loose the connection with them,
which could lead to data corruption. See http://lwn.net/Articles/171941/
for details.

2. The interface assumes that all allocated memory by the user space
handler is DMA'able by the target hardware. This is almost always true
for most modern systems, except if the target hardware isn't capable of
using 64-bit address space and the system has >4GB of memory or the
memory addresses are in address space, which is unavailable with 32-bit
addresses.

In case of any error the error reporting should be deferred until
SCST_USER_EXEC subcommand, where the appropriate SAM status and sense
should be set.


		4.4.5. SCST_USER_EXEC

SCST_USER_EXEC returns SCSI command on execution state of the SCST
processing. The user space handler should execute the SCSI command and
reply using the corresponding reply command.

In some cases for performance reasons for READ-type SCSI commands
SCST_USER_ALLOC_MEM subcommand isn't returned before SCST_USER_EXEC.
Thus, if pbuf pointer is 0 and the SCSI command needs data transfer,
the user space handler should be prepared to allocate the data buffer
with size alloc_len, which could be up to 2 times more, than actually
required by the SCSI command. But field bufflen will contain the correct
value. All the memory reusage rules, described for SCST_USER_ALLOC_MEM,
apply to SCST_USER_EXEC as well.

Payload contains struct scst_user_scsi_cmd_exec, which is defined as the
following:

struct scst_user_scsi_cmd_exec
{
	uint64_t sess_h;

	uint8_t cdb[SCST_MAX_CDB_SIZE];
	int32_t cdb_len;

	int32_t data_len;
	int32_t bufflen;
	int32_t alloc_len;
	uint64_t pbuf;
	uint8_t queue_type;
	uint8_t data_direction;
	uint8_t partial;
	uint32_t timeout;

	uint32_t sn;

	uint32_t parent_cmd_h;
	int32_t parent_cmd_data_len;
	uint32_t partial_offset;
},

where:

 - sess_h - corresponding session handler

 - cdb - SCSI CDB

 - cdb_len - SCSI CDB length

 - data_len - command's data length. Could be different from bufflen for
   commands like VERIFY, which transfer different amount of data, than
   process, or even none of them
 
 - bufflen - command's buffer length

 - alloc_len - command's buffer length, which should be allocated, if pbuf
   is 0 and the command requires data transfer

 - pbuf - pointer to command's data buffer or 0 for SCSI commands
   without data transfer.

 - queue_type - SCSI task attribute (queue type)

 - data_direction - command's data flow direction, one of SCST_DATA_*
   constants

 - partial - specifies, if the command is a partial subcommand, could
   have the following OR'ed flags:

   * SCST_USER_SUBCOMMAND - set if the command is a partial subcommand
   
   * SCST_USER_SUBCOMMAND_FINAL - set if the subcommand is a final one

 - timeout - CDB execution timeout

 - sn - command's SN, which might be used for task management

 - parent_cmd_h - has the same unique value for all partial data
   transfers subcommands of one original (parent) command

 - parent_cmd_data_len - for partial data transfers subcommand has the
   size of the overall data transfer of the original (parent) command

 - partial_offset - has offset of the subcommand in the original
   (parent) command

It is guaranteed that only commands of the same queue_type per session
can be returned simultaneously.

In case of any error it should be reported via appropriate SAM status
and sense. If it happens for a subcommand of a partial data transfers
command, all other subcommands of this command, which already passed the
the user space handler or will be passed in the future, will be aborted
by scst_user, the user space handler should ignore them.


		4.4.6. SCST_USER_ON_FREE_CMD

SCST_USER_ON_FREE_CMD returns SCSI command when the command is about to
be freed. At this stage, the user space device handler could do any
necessary cleanups, for instance, free allocated for data buffer memory.

NOTE! If the memory reusage is enabled, then the data buffer must not be
freed, it will be reused by subsequent SCSI commands. The buffer must be
freed only on SCST_USER_ON_CACHED_MEM_FREE event.

Payload contains struct scst_user_scsi_on_free_cmd, which is defined
as the following:

struct scst_user_scsi_on_free_cmd
{
	uint64_t pbuf;
	int32_t resp_data_len;
	uint8_t buffer_cached;
	uint8_t aborted;
	uint8_t status;
	uint8_t delivery_status;
},

where:

 - pbuf - pointer to command's data buffer or 0 for SCSI commands
   without data transfer.

 - resp_data_len - length of the response data

 - buffer_cached - true, if memory reusage is enabled for this command

 - aborted - true, if command was aborted

 - status - SAM status of the commands execution

 - delivery_status - status of cmd's status/data delivery to remote
   initiator. Can be:
   
   * SCST_CMD_DELIVERY_SUCCESS - delivery succeeded

   * SCST_CMD_DELIVERY_FAILED - delivery failed

The user space handler should reply using the corresponding reply
command. No error code is needed.


		4.4.7. SCST_USER_ON_CACHED_MEM_FREE

SCST_USER_ON_CACHED_MEM_FREE subcommand is returned, when SGV cache
decided that this buffer isn't needed anymore. This happens after some
time of inactivity or when the system is under memory pressure.

Payload contains struct scst_user_on_cached_mem_free, which is defined
as the following:

struct scst_user_scsi_cmd_alloc_mem
{
	uint64_t pbuf;
},

where:

 - pbuf - pointer to buffer, which should be freed


		4.4.8. SCST_USER_TASK_MGMT
 
SCST_USER_TASK_MGMT subcommand returns a task management functions.
Payload contains struct scst_user_tm, which is defined as the following:

struct scst_user_tm
{
	uint64_t sess_h;
	uint32_t fn;
	uint32_t cmd_h_to_abort;
	uint32_t cmd_sn;
	uint8_t cmd_sn_set;
},

where:

 - sess_h - corresponding session handler

 - fn - task management function, see below

 - cmd_h_to_abort - handle of command to abort. Valid only if fn is
   SCST_ABORT_TASK

 - cmd_sn - if cmd_sn_set is set, contains maximum commands SN, which
   this task management function affects. See iSCSI RFC 3720 10.5.1 for
   more details.

 - cmd_sn_set - specifies if cmd_sn is valid
   
After TM function is completed, the user space device handler shall
reply using "result" field of the corresponding reply command. It isn't
necessary to wait for aborted command(s) finished, the result of TM
function shall be returned immediately. SCST core will take care that
the reply to the TM function isn't sent before all affection SCSI
commands finished.

Possible values of "fn" field:

 - SCST_ABORT_TASK - cmd_h_to_abort shall be aborted

 - SCST_ABORT_TASK_SET - task set on the device shall be aborted

 - SCST_CLEAR_ACA - ACA status shall be cleared

 - SCST_CLEAR_TASK_SET  - task set on the device shall be cleared

 - SCST_LUN_RESET, SCST_TARGET_RESET - reset of the device shall be done

 - SCST_NEXUS_LOSS_SESS - notifies about nexus loss event for the session

 - SCST_ABORT_ALL_TASKS_SESS - all tasks in the session shall be aborted
 
 - SCST_NEXUS_LOSS - notifies about global nexus loss event
 
 - SCST_ABORT_ALL_TASKS - all tasks shall be aborted

Possible return values are:

 - SCST_MGMT_STATUS_SUCCESS - success

 - SCST_MGMT_STATUS_TASK_NOT_EXIST - task does not exist

 - SCST_MGMT_STATUS_LUN_NOT_EXIST - LUN does not exist

 - SCST_MGMT_STATUS_FN_NOT_SUPPORTED - task management function not
   supported

 - SCST_MGMT_STATUS_REJECTED - task management function was rejected

 - SCST_MGMT_STATUS_FAILED - task management function failed


		5. SCST_USER_REPLY_CMD

SCST_USER_REPLY_CMD IOCTL function allows the user space handler to
return the result of a command's execution. Its argument is defined as:

struct scst_user_reply_cmd
{
	uint32_t cmd_h;
	uint32_t subcode;
	union {
		int32_t result;
		struct scst_user_scsi_cmd_reply_parse parse_reply;
		struct scst_user_scsi_cmd_reply_alloc_mem alloc_reply;
		struct scst_user_scsi_cmd_reply_exec exec_reply;
	};
},

where:

 - cmd_h - command handle used to identify the command in the reply.

 - subcode - subcommand code, see 4.1

Union contains the subcommand's specific payload:

 - result - subcommand's result code


struct scst_user_scsi_cmd_reply_parse
{
	uint8_t queue_type;
	uint8_t data_direction;
	int32_t data_len;
	int32_t bufflen;
},

where:

 - queue_type - SCSI task attribute (queue type )
 
 - data_direction - command's data flow direction, one of SCST_DATA_* constants

 - data_len - command's data length
 
 - bufflen - command's buffer length


struct scst_user_scsi_cmd_reply_alloc_mem
{
	uint64_t pbuf;
},

where:

 - pbuf - pointer to command's data buffer


struct scst_user_scsi_cmd_reply_exec
{
	int32_t resp_data_len;
	uint64_t pbuf;

	uint8_t reply_type;

	uint8_t status;
	uint8_t sense_len;
	aligned_u64 psense_buffer;
},

where:

 - resp_data_len - length of the response data

 - pbuf - pointer to command's data buffer. Used only when in the
   original SCST_USER_EXEC subcommand pbuf field is 0

 - reply_type - could be one of the following constants:
 
     * SCST_EXEC_REPLY_BACKGROUND - tells SCST send to the remote
       initiator GOOD status, but the command not yet completed by the
       user space handler, it is being executed in the background. When
       it will be completed, the user space handler will call
       SCST_USER_REPLY_CMD again with reply_type
       SCST_EXEC_REPLY_COMPLETED.
     
     * SCST_EXEC_REPLY_COMPLETED - the user space handler completed the command

 - status - SAM status of the commands execution

 - sense_len - length of sense data in psense_buffer, if any

 - psense_buffer - pointed to sense buffer


		6. SCST_USER_FLUSH_CACHE

SCST_USER_FLUSH_CACHE - flushes SGV cache for the corresponding virtual
user space device and queues for all cached memory buffers corresponding
SCST_USER_ON_CACHED_MEM_FREE subcommands.

During execution of SCST_USER_FLUSH_CACHE at least one another thread
must process all coming subcommands, otherwise after timeout it will
fail with EBUSY error.

SCST_USER_FLUSH_CACHE doesn't have any parameters.

SCST_USER_FLUSH_CACHE returns 0 on success or -1 in case of error,
and errno is set appropriately.


		7. SCST_USER_DEVICE_CAPACITY_CHANGED

SCST_USER_DEVICE_CAPACITY_CHANGED - queues CAPACITY DATA HAS CHANGED
Unit Attention or corresponding Asynchronous Event to the corresponding
virtual device. It will notify remote initiators, connected to the
device, and allow them to automatically refresh new device size. You
should use SCST_USER_DEVICE_CAPACITY_CHANGED after resize of the device.

SCST_USER_DEVICE_CAPACITY_CHANGED doesn't have any parameters.

SCST_USER_DEVICE_CAPACITY_CHANGED returns 0 on success or -1 in case of
error, and errno is set appropriately.


		IV. Commands processing flow example.

As the example consider a simple synchronous VTL, which serves one
virtual SCSI tape device and can process only one command at time from
any initiator. 

At the beginning the VTL opens using open() call /dev/scst_user in the
default blocking mode.

Then it using SCST_USER_REGISTER_DEVICE ioctl() function registers the
tape device. Since only one command at time is supported, the allocated
command's data memory could be reused for both READ-type (i.e. which
involve data transfer from target to initiator) and WRITE-type (i.e.
which involve data transfer from initiator to target) commands. So the
device is configured with parse_type SCST_USER_PARSE_STANDARD,
on_free_cmd_type SCST_USER_ON_FREE_CMD_IGNORE, memory_reuse_type
SCST_USER_MEM_REUSE_ALL and partial_transfers_type
SCST_USER_PARTIAL_TRANSFERS_NOT_SUPPORTED.

Then it prepares struct scst_user_get_cmd with reply set to 0, calls
SCST_USER_REPLY_AND_GET_CMD ioctl() and waits until some initiator
connects to its tape device. On that event the VTL receives
SCST_USER_ATTACH_SESS subcommand. Since the VTL doesn't use any
initiator specific data, it can do nothing on that subcommand, so it
prepares scst_user_reply_cmd structure, where:

 - cmd_h set to returned by SCST_USER_REPLY_AND_GET_CMD ioctl() cmd_h

 - subcode set to SCST_USER_ATTACH_SESS
 
 - result set to 0
 
Then it prepares struct scst_user_get_cmd with reply set to the prepared
scst_user_reply_cmd structure, calls SCST_USER_REPLY_AND_GET_CMD ioctl()
and waits for some SCSI command arrives from the initiator.

If the received SCSI command is READ-type one, SCST does the necessary
preparations, then the VTL receives SCST_USER_EXEC subcommand, where
bufflen and data_len are set correctly, but memory for buffer isn't
allocated, so pbuf field is 0. The VTL then allocates the data buffer
with size alloc_len, e.g. using malloc(). Then the VTL reads the data
from disk in it, e.g. using O_DIRECT read() function, then prepares
scst_user_reply_cmd structure, where:

 - cmd_h set to returned by SCST_USER_REPLY_AND_GET_CMD ioctl() cmd_h

 - subcode set to SCST_USER_EXEC

 - exec_reply.resp_data_len set to length of the read data

 - exec_reply.pbuf set to the data buffer, where the data were read
 
 - exec_reply.reply_type set to SCST_EXEC_REPLY_COMPLETED
 
 - exec_reply.status set to the SAM defined status of the
   operation
 
 - exec_reply.sense_len set and exec_reply.psense_buffer filled with
   sense data, if necessary

Then it prepares struct scst_user_get_cmd with reply set to the prepared
scst_user_reply_cmd structure, calls SCST_USER_REPLY_AND_GET_CMD ioctl()
and waits for the next SCSI command arrives from the initiator. That's
all for this SCSI command. For the next one the used data buffer will be
reused.

For WRITE-type SCSI commands the processing is the same, but
SCST_USER_ALLOC_MEM will be returned before SCST_USER_EXEC, since the
data transfer from the initiator precedes the commands execution.

In case, if the first command requires 4K data buffer, but the second
one - 1M, for it the VTL also will be asked to allocate the buffer.
Then, if no more 4K commands come for some time, for it
SCST_USER_ON_CACHED_MEM_FREE subcommand will be returned to the VTL in
order to ask it to free that buffer.

Vladislav Bolkhovitin