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diff --git a/Documentation/arch/s390/s390dbf.rst b/Documentation/arch/s390/s390dbf.rst new file mode 100644 index 0000000000..af8bdc3629 --- /dev/null +++ b/Documentation/arch/s390/s390dbf.rst @@ -0,0 +1,478 @@ +================== +S390 Debug Feature +================== + +files: + - arch/s390/kernel/debug.c + - arch/s390/include/asm/debug.h + +Description: +------------ +The goal of this feature is to provide a kernel debug logging API +where log records can be stored efficiently in memory, where each component +(e.g. device drivers) can have one separate debug log. +One purpose of this is to inspect the debug logs after a production system crash +in order to analyze the reason for the crash. + +If the system still runs but only a subcomponent which uses dbf fails, +it is possible to look at the debug logs on a live system via the Linux +debugfs filesystem. + +The debug feature may also very useful for kernel and driver development. + +Design: +------- +Kernel components (e.g. device drivers) can register themselves at the debug +feature with the function call :c:func:`debug_register()`. +This function initializes a +debug log for the caller. For each debug log exists a number of debug areas +where exactly one is active at one time. Each debug area consists of contiguous +pages in memory. In the debug areas there are stored debug entries (log records) +which are written by event- and exception-calls. + +An event-call writes the specified debug entry to the active debug +area and updates the log pointer for the active area. If the end +of the active debug area is reached, a wrap around is done (ring buffer) +and the next debug entry will be written at the beginning of the active +debug area. + +An exception-call writes the specified debug entry to the log and +switches to the next debug area. This is done in order to be sure +that the records which describe the origin of the exception are not +overwritten when a wrap around for the current area occurs. + +The debug areas themselves are also ordered in form of a ring buffer. +When an exception is thrown in the last debug area, the following debug +entries are then written again in the very first area. + +There are four versions for the event- and exception-calls: One for +logging raw data, one for text, one for numbers (unsigned int and long), +and one for sprintf-like formatted strings. + +Each debug entry contains the following data: + +- Timestamp +- Cpu-Number of calling task +- Level of debug entry (0...6) +- Return Address to caller +- Flag, if entry is an exception or not + +The debug logs can be inspected in a live system through entries in +the debugfs-filesystem. Under the toplevel directory "``s390dbf``" there is +a directory for each registered component, which is named like the +corresponding component. The debugfs normally should be mounted to +``/sys/kernel/debug`` therefore the debug feature can be accessed under +``/sys/kernel/debug/s390dbf``. + +The content of the directories are files which represent different views +to the debug log. Each component can decide which views should be +used through registering them with the function :c:func:`debug_register_view()`. +Predefined views for hex/ascii and sprintf data are provided. +It is also possible to define other views. The content of +a view can be inspected simply by reading the corresponding debugfs file. + +All debug logs have an actual debug level (range from 0 to 6). +The default level is 3. Event and Exception functions have a :c:data:`level` +parameter. Only debug entries with a level that is lower or equal +than the actual level are written to the log. This means, when +writing events, high priority log entries should have a low level +value whereas low priority entries should have a high one. +The actual debug level can be changed with the help of the debugfs-filesystem +through writing a number string "x" to the ``level`` debugfs file which is +provided for every debug log. Debugging can be switched off completely +by using "-" on the ``level`` debugfs file. + +Example:: + + > echo "-" > /sys/kernel/debug/s390dbf/dasd/level + +It is also possible to deactivate the debug feature globally for every +debug log. You can change the behavior using 2 sysctl parameters in +``/proc/sys/s390dbf``: + +There are currently 2 possible triggers, which stop the debug feature +globally. The first possibility is to use the ``debug_active`` sysctl. If +set to 1 the debug feature is running. If ``debug_active`` is set to 0 the +debug feature is turned off. + +The second trigger which stops the debug feature is a kernel oops. +That prevents the debug feature from overwriting debug information that +happened before the oops. After an oops you can reactivate the debug feature +by piping 1 to ``/proc/sys/s390dbf/debug_active``. Nevertheless, it's not +suggested to use an oopsed kernel in a production environment. + +If you want to disallow the deactivation of the debug feature, you can use +the ``debug_stoppable`` sysctl. If you set ``debug_stoppable`` to 0 the debug +feature cannot be stopped. If the debug feature is already stopped, it +will stay deactivated. + +Kernel Interfaces: +------------------ + +.. kernel-doc:: arch/s390/kernel/debug.c +.. kernel-doc:: arch/s390/include/asm/debug.h + +Predefined views: +----------------- + +.. code-block:: c + + extern struct debug_view debug_hex_ascii_view; + + extern struct debug_view debug_sprintf_view; + +Examples +-------- + +.. code-block:: c + + /* + * hex_ascii-view Example + */ + + #include <linux/init.h> + #include <asm/debug.h> + + static debug_info_t *debug_info; + + static int init(void) + { + /* register 4 debug areas with one page each and 4 byte data field */ + + debug_info = debug_register("test", 1, 4, 4 ); + debug_register_view(debug_info, &debug_hex_ascii_view); + + debug_text_event(debug_info, 4 , "one "); + debug_int_exception(debug_info, 4, 4711); + debug_event(debug_info, 3, &debug_info, 4); + + return 0; + } + + static void cleanup(void) + { + debug_unregister(debug_info); + } + + module_init(init); + module_exit(cleanup); + +.. code-block:: c + + /* + * sprintf-view Example + */ + + #include <linux/init.h> + #include <asm/debug.h> + + static debug_info_t *debug_info; + + static int init(void) + { + /* register 4 debug areas with one page each and data field for */ + /* format string pointer + 2 varargs (= 3 * sizeof(long)) */ + + debug_info = debug_register("test", 1, 4, sizeof(long) * 3); + debug_register_view(debug_info, &debug_sprintf_view); + + debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__); + debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info); + + return 0; + } + + static void cleanup(void) + { + debug_unregister(debug_info); + } + + module_init(init); + module_exit(cleanup); + +Debugfs Interface +----------------- +Views to the debug logs can be investigated through reading the corresponding +debugfs-files: + +Example:: + + > ls /sys/kernel/debug/s390dbf/dasd + flush hex_ascii level pages + > cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s + 00 00974733272:680099 2 - 02 0006ad7e 07 ea 4a 90 | .... + 00 00974733272:682210 2 - 02 0006ade6 46 52 45 45 | FREE + 00 00974733272:682213 2 - 02 0006adf6 07 ea 4a 90 | .... + 00 00974733272:682281 1 * 02 0006ab08 41 4c 4c 43 | EXCP + 01 00974733272:682284 2 - 02 0006ab16 45 43 4b 44 | ECKD + 01 00974733272:682287 2 - 02 0006ab28 00 00 00 04 | .... + 01 00974733272:682289 2 - 02 0006ab3e 00 00 00 20 | ... + 01 00974733272:682297 2 - 02 0006ad7e 07 ea 4a 90 | .... + 01 00974733272:684384 2 - 00 0006ade6 46 52 45 45 | FREE + 01 00974733272:684388 2 - 00 0006adf6 07 ea 4a 90 | .... + +See section about predefined views for explanation of the above output! + +Changing the debug level +------------------------ + +Example:: + + + > cat /sys/kernel/debug/s390dbf/dasd/level + 3 + > echo "5" > /sys/kernel/debug/s390dbf/dasd/level + > cat /sys/kernel/debug/s390dbf/dasd/level + 5 + +Flushing debug areas +-------------------- +Debug areas can be flushed with piping the number of the desired +area (0...n) to the debugfs file "flush". When using "-" all debug areas +are flushed. + +Examples: + +1. Flush debug area 0:: + + > echo "0" > /sys/kernel/debug/s390dbf/dasd/flush + +2. Flush all debug areas:: + + > echo "-" > /sys/kernel/debug/s390dbf/dasd/flush + +Changing the size of debug areas +------------------------------------ +It is possible the change the size of debug areas through piping +the number of pages to the debugfs file "pages". The resize request will +also flush the debug areas. + +Example: + +Define 4 pages for the debug areas of debug feature "dasd":: + + > echo "4" > /sys/kernel/debug/s390dbf/dasd/pages + +Stopping the debug feature +-------------------------- +Example: + +1. Check if stopping is allowed:: + + > cat /proc/sys/s390dbf/debug_stoppable + +2. Stop debug feature:: + + > echo 0 > /proc/sys/s390dbf/debug_active + +crash Interface +---------------- +The ``crash`` tool since v5.1.0 has a built-in command +``s390dbf`` to display all the debug logs or export them to the file system. +With this tool it is possible +to investigate the debug logs on a live system and with a memory dump after +a system crash. + +Investigating raw memory +------------------------ +One last possibility to investigate the debug logs at a live +system and after a system crash is to look at the raw memory +under VM or at the Service Element. +It is possible to find the anchor of the debug-logs through +the ``debug_area_first`` symbol in the System map. Then one has +to follow the correct pointers of the data-structures defined +in debug.h and find the debug-areas in memory. +Normally modules which use the debug feature will also have +a global variable with the pointer to the debug-logs. Following +this pointer it will also be possible to find the debug logs in +memory. + +For this method it is recommended to use '16 * x + 4' byte (x = 0..n) +for the length of the data field in :c:func:`debug_register()` in +order to see the debug entries well formatted. + + +Predefined Views +---------------- + +There are two predefined views: hex_ascii and sprintf. +The hex_ascii view shows the data field in hex and ascii representation +(e.g. ``45 43 4b 44 | ECKD``). + +The sprintf view formats the debug entries in the same way as the sprintf +function would do. The sprintf event/exception functions write to the +debug entry a pointer to the format string (size = sizeof(long)) +and for each vararg a long value. So e.g. for a debug entry with a format +string plus two varargs one would need to allocate a (3 * sizeof(long)) +byte data area in the debug_register() function. + +IMPORTANT: + Using "%s" in sprintf event functions is dangerous. You can only + use "%s" in the sprintf event functions, if the memory for the passed string + is available as long as the debug feature exists. The reason behind this is + that due to performance considerations only a pointer to the string is stored + in the debug feature. If you log a string that is freed afterwards, you will + get an OOPS when inspecting the debug feature, because then the debug feature + will access the already freed memory. + +NOTE: + If using the sprintf view do NOT use other event/exception functions + than the sprintf-event and -exception functions. + +The format of the hex_ascii and sprintf view is as follows: + +- Number of area +- Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated + Universal Time (UTC), January 1, 1970) +- level of debug entry +- Exception flag (* = Exception) +- Cpu-Number of calling task +- Return Address to caller +- data field + +A typical line of the hex_ascii view will look like the following (first line +is only for explanation and will not be displayed when 'cating' the view):: + + area time level exception cpu caller data (hex + ascii) + -------------------------------------------------------------------------- + 00 00964419409:440690 1 - 00 88023fe + + +Defining views +-------------- + +Views are specified with the 'debug_view' structure. There are defined +callback functions which are used for reading and writing the debugfs files: + +.. code-block:: c + + struct debug_view { + char name[DEBUG_MAX_PROCF_LEN]; + debug_prolog_proc_t* prolog_proc; + debug_header_proc_t* header_proc; + debug_format_proc_t* format_proc; + debug_input_proc_t* input_proc; + void* private_data; + }; + +where: + +.. code-block:: c + + typedef int (debug_header_proc_t) (debug_info_t* id, + struct debug_view* view, + int area, + debug_entry_t* entry, + char* out_buf); + + typedef int (debug_format_proc_t) (debug_info_t* id, + struct debug_view* view, char* out_buf, + const char* in_buf); + typedef int (debug_prolog_proc_t) (debug_info_t* id, + struct debug_view* view, + char* out_buf); + typedef int (debug_input_proc_t) (debug_info_t* id, + struct debug_view* view, + struct file* file, const char* user_buf, + size_t in_buf_size, loff_t* offset); + + +The "private_data" member can be used as pointer to view specific data. +It is not used by the debug feature itself. + +The output when reading a debugfs file is structured like this:: + + "prolog_proc output" + + "header_proc output 1" "format_proc output 1" + "header_proc output 2" "format_proc output 2" + "header_proc output 3" "format_proc output 3" + ... + +When a view is read from the debugfs, the Debug Feature calls the +'prolog_proc' once for writing the prolog. +Then 'header_proc' and 'format_proc' are called for each +existing debug entry. + +The input_proc can be used to implement functionality when it is written to +the view (e.g. like with ``echo "0" > /sys/kernel/debug/s390dbf/dasd/level``). + +For header_proc there can be used the default function +:c:func:`debug_dflt_header_fn()` which is defined in debug.h. +and which produces the same header output as the predefined views. +E.g:: + + 00 00964419409:440761 2 - 00 88023ec + +In order to see how to use the callback functions check the implementation +of the default views! + +Example: + +.. code-block:: c + + #include <asm/debug.h> + + #define UNKNOWNSTR "data: %08x" + + const char* messages[] = + {"This error...........\n", + "That error...........\n", + "Problem..............\n", + "Something went wrong.\n", + "Everything ok........\n", + NULL + }; + + static int debug_test_format_fn( + debug_info_t *id, struct debug_view *view, + char *out_buf, const char *in_buf + ) + { + int i, rc = 0; + + if (id->buf_size >= 4) { + int msg_nr = *((int*)in_buf); + if (msg_nr < sizeof(messages) / sizeof(char*) - 1) + rc += sprintf(out_buf, "%s", messages[msg_nr]); + else + rc += sprintf(out_buf, UNKNOWNSTR, msg_nr); + } + return rc; + } + + struct debug_view debug_test_view = { + "myview", /* name of view */ + NULL, /* no prolog */ + &debug_dflt_header_fn, /* default header for each entry */ + &debug_test_format_fn, /* our own format function */ + NULL, /* no input function */ + NULL /* no private data */ + }; + +test: +===== + +.. code-block:: c + + debug_info_t *debug_info; + int i; + ... + debug_info = debug_register("test", 0, 4, 4); + debug_register_view(debug_info, &debug_test_view); + for (i = 0; i < 10; i ++) + debug_int_event(debug_info, 1, i); + +:: + + > cat /sys/kernel/debug/s390dbf/test/myview + 00 00964419734:611402 1 - 00 88042ca This error........... + 00 00964419734:611405 1 - 00 88042ca That error........... + 00 00964419734:611408 1 - 00 88042ca Problem.............. + 00 00964419734:611411 1 - 00 88042ca Something went wrong. + 00 00964419734:611414 1 - 00 88042ca Everything ok........ + 00 00964419734:611417 1 - 00 88042ca data: 00000005 + 00 00964419734:611419 1 - 00 88042ca data: 00000006 + 00 00964419734:611422 1 - 00 88042ca data: 00000007 + 00 00964419734:611425 1 - 00 88042ca data: 00000008 + 00 00964419734:611428 1 - 00 88042ca data: 00000009 |