[PATCH v4 10/10] user_events: Add documentation file

[Beau Belgrave]

Add a documentation file about user_events with example code, etc.
explaining how it may be used.

Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
---
 Documentation/trace/user_events.rst | 298 ++++++++++++++++++++++++++++
 1 file changed, 298 insertions(+)
 create mode 100644 Documentation/trace/user_events.rst

diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
new file mode 100644
index 000000000000..d79c9f07d012
--- /dev/null
+++ b/Documentation/trace/user_events.rst
@@ -0,0 +1,298 @@
+=========================================
+user_events: User-based Event Tracing
+=========================================
+
+:Author: Beau Belgrave
+
+Overview
+--------
+User based trace events allow user processes to create events and trace data
+that can be viewed via existing tools, such as ftrace, perf and eBPF.
+To enable this feature, build your kernel with CONFIG_USER_EVENTS=y.
+
+Programs can view status of the events via 
+/sys/kernel/debug/tracing/user_events_status and can both register and write
+data out via /sys/kernel/debug/tracing/user_events_data.
+
+Programs can also use /sys/kernel/debug/tracing/dynamic_events to register and
+delete user based events via the u: prefix. The format of the command to
+dynamic_events is the same as the ioctl with the u: prefix applied.
+
+Typically programs will register a set of events that they wish to expose to
+tools that can read trace_events (such as ftrace and perf). The registration
+process gives back two ints to the program for each event. The first int is the
+status index. This index describes which byte in the 
+/sys/kernel/debug/tracing/user_events_status file represents this event. The
+second int is the write index. This index describes the data when a write() or
+writev() is called on the /sys/kernel/debug/tracing/user_events_data file.
+
+The structures referenced in this document are contained with the
+/include/uap/linux/user_events.h file in the source tree.
+
+**NOTE:** *Both user_events_status and user_events_data are under the tracefs filesystem
+and may be mounted at different paths than above.*
+
+Registering
+-----------
+Registering within a user process is done via ioctl() out to the
+/sys/kernel/debug/tracing/user_events_data file. The command to issue is
+DIAG_IOCSREG. This command takes a struct user_reg as an argument.
+
+The struct user_reg requires two values, the first is the size of the structure
+to ensure forward and backward compatibility. The second is the command string
+to issue for registering.
+
+User based events show up under tracefs like any other event under the subsystem
+named "user_events". This means tools that wish to attach to the events need to
+use /sys/kernel/debug/tracing/events/user_events/[name]/enable or perf record
+-e user_events:[name] when attaching/recording.
+
+**NOTE:** *The write_index returned is only valid for the FD that was used*
+
+Command Format
+^^^^^^^^^^^^^^
+The command string format is as follows:
+
+::
+
+  name[:FLAG1[,FLAG2...]] [Field1[;Field2...]]
+
+Supported Flags
+^^^^^^^^^^^^^^^
+**BPF_ITER** - EBPF programs attached to this event will get the raw iovec
+struct instead of any data copies for max performance.
+
+Field Format
+^^^^^^^^^^^^
+
+::
+
+  type name [size]
+
+Basic types are supported (__data_loc, u32, u64, int, char, char[20]).
+User programs are encouraged to use clearly sized types like u32.
+
+**NOTE:** *Long is not supported since size can vary between user and kernel.*
+
+The size is only valid for types that start with a struct prefix.
+This allows user programs to describe custom structs out to tools, if required.
+
+For example, a struct in C that looks like this:
+
+::
+
+  struct mytype {
+    char data[20];
+  };
+
+Would be represented by the following field:
+
+::
+
+  struct mytype myname 20
+
+Status
+------
+When tools attach/record user based events the status of the event is updated
+in realtime. This allows user programs to only incur the cost of the write() or
+writev() calls when something is actively attached to the event.
+
+User programs call mmap() on /sys/kernel/debug/tracing/user_events_status to
+check the status for each event that is registered. The byte to check in the
+file is given back after the register ioctl() via user_reg.status_index.
+Currently the size of user_events_status is a single page, however, custom
+kernel configurations can change this size to allow more user based events. In
+all cases the size of the file is a multiple of a page size.
+
+For example, if the register ioctl() gives back a status_index of 3 you would
+check byte 3 of the returned mmap data to see if anything is attached to that
+event.
+
+Administrators can easily check the status of all registered events by reading
+the user_events_status file directly via a terminal. The output is as follows:
+
+::
+
+  Byte:Name [# Comments]
+  ...
+
+  Active: ActiveCount
+  Buisy: BusyCount
+  Max: MaxCount
+
+For example, on a system that has a single event the output looks like this:
+
+::
+
+  1:test
+
+  Active: 1
+  Busy: 0
+  Max: 4096
+
+If a user enables the user event via ftrace, the output would change to this:
+
+:: 
+
+  1:test # Used by ftrace
+
+  Active: 1
+  Busy: 1
+  Max: 4096
+
+**NOTE:** *A status index of 0 will never be returned. This allows user 
+programs to have an index that can be used on error cases.*
+
+Status Bits
+^^^^^^^^^^^
+The byte being checked will be non-zero if anything is attached. Programs can
+check specific bits in the byte to see what mechanism has been attached.
+
+The following values are defined to aid in checking what has been attached:
+**EVENT_STATUS_FTRACE** - Bit set if ftrace has been attached (Bit 0).
+
+**EVENT_STATUS_PERF** - Bit set if perf/eBPF has been attached (Bit 1).
+
+Writing Data
+------------
+After registering an event the same fd that was used to register can be used
+to write an entry for that event. The write_index returned must be at the start
+of the data, then the remaining data is treated as the payload of the event.
+
+For example, if write_index returned was 1 and I wanted to write out an int
+payload of the event. Then the data would have to be 8 bytes (2 ints) long,
+with the first 4 bytes being equal to 1 and the last 4 bytes being equal to the
+value I want as the payload.
+
+In memory this would look like this:
+
+::
+
+  int index;
+  int payload;
+
+User programs might have well known structs that they wish to use to emit out
+as payloads. In those cases writev() can be used, with the first vector being
+the index and the following vector(s) being the actual event payload.
+
+For example, if I have a struct like this:
+
+::
+
+  struct payload {
+        int src;
+        int dst;
+        int flags;
+  };
+
+It's advised for user programs to do the following:
+
+:: 
+
+  struct iovec io[2];
+  struct payload e;
+
+  io[0].iov_base = &write_index;
+  io[0].iov_len = sizeof(write_index);
+  io[1].iov_base = &e;
+  io[1].iov_len = sizeof(e);
+
+  writev(fd, (const struct iovec*)io, 2);
+
+**NOTE:** *The write_index is not emitted out into the trace being recorded.*
+
+EBPF
+----
+EBPF programs that attach to a user-based event tracepoint are given a pointer
+to a struct user_bpf_context. The bpf context contains the data type (which can
+be a user or kernel buffer, or can be a pointer to the iovec) and the data
+length that was emitted (minus the write_index).
+
+Example Code
+------------
+
+::
+
+  #include <errno.h>
+  #include <sys/ioctl.h>
+  #include <sys/mman.h>
+  #include <fcntl.h>
+  #include <stdio.h>
+  #include <unistd.h>
+  #include <linux/user_events.h>
+  
+  /* Assumes debugfs is mounted */
+  const char *data_file = "/sys/kernel/debug/tracing/user_events_data";
+  const char *status_file = "/sys/kernel/debug/tracing/user_events_status";
+  
+  static int event_status(char **status)
+  {
+  	int fd = open(status_file, O_RDONLY);
+  
+  	*status = mmap(NULL, sysconf(_SC_PAGESIZE), PROT_READ,
+  		       MAP_SHARED, fd, 0);
+  
+  	close(fd);
+  
+  	if (*status == MAP_FAILED)
+  	      return -1;
+  
+  	return 0;
+  }
+  
+  static int event_reg(int fd, const char *command, int *status, int *write)
+  {
+  	struct user_reg reg = {0};
+  
+  	reg.size = sizeof(reg);
+  	reg.name_args = (__u64)command;
+  
+  	if (ioctl(fd, DIAG_IOCSREG, &reg) == -1)
+  		return -1;
+  
+  	*status = reg.status_index;
+  	*write = reg.write_index;
+  
+  	return 0;
+  }
+  
+  int main(int argc, char **argv)
+  {
+  	int data_fd, status, write;
+  	char *status_page;
+  	struct iovec io[2];
+  	__u32 count = 0;
+  
+  	if (event_status(&status_page) == -1)
+  		return errno;
+  
+  	data_fd = open(data_file, O_RDWR);
+  
+  	if (event_reg(data_fd, "test u32 count", &status, &write) == -1)
+  		return errno;
+  
+  	/* Setup iovec */
+  	io[0].iov_base = &status;
+  	io[0].iov_len = sizeof(status);
+  	io[1].iov_base = &count;
+  	io[1].iov_len = sizeof(count);
+  
+  ask:
+  	printf("Press enter to check status...\n");
+  	getchar();
+  
+  	/* Check if anyone is listening */
+  	if (status_page[status]) {
+  		/* Yep, trace out our data */
+  		writev(data_fd, (const struct iovec*)io, 2);
+  
+  		/* Increase the count */
+  		count++;
+  
+  		printf("Something was attached, wrote data\n");
+  	}
+  
+  	goto ask;
+  
+  	return 0;
+  }
-- 
2.17.1