perf tools: Add documentation for BPF event selection

Add documentation for how to pass a BPF program as a perf event.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190201134651.12373-1-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -120,6 +120,10 @@ Given a $HOME/.perfconfig like this:
 		children = true
 		group = true
 
+	[llvm]
+		dump-obj = true
+		clang-opt = -g
+
 You can hide source code of annotate feature setting the config to false with
 
 	% perf config annotate.hide_src_code=true
@@ -553,6 +557,33 @@ trace.*::
 	trace.show_zeros::
 		Do not suppress syscall arguments that are equal to zero.
 
+llvm.*::
+	llvm.clang-path::
+		Path to clang. If omit, search it from $PATH.
+
+	llvm.clang-bpf-cmd-template::
+		Cmdline template. Below lines show its default value. Environment
+		variable is used to pass options.
+		"$CLANG_EXEC -D__KERNEL__ $CLANG_OPTIONS $KERNEL_INC_OPTIONS \
+		-Wno-unused-value -Wno-pointer-sign -working-directory \
+		$WORKING_DIR  -c $CLANG_SOURCE -target bpf -O2 -o -"
+
+	llvm.clang-opt::
+		Options passed to clang.
+
+	llvm.kbuild-dir::
+		kbuild directory. If not set, use /lib/modules/`uname -r`/build.
+		If set to "" deliberately, skip kernel header auto-detector.
+
+	llvm.kbuild-opts::
+		Options passed to 'make' when detecting kernel header options.
+
+	llvm.dump-obj::
+		Enable perf dump BPF object files compiled by LLVM.
+
+	llvm.opts::
+		Options passed to llc.
+
 SEE ALSO
 --------
 linkperf:perf[1]

tools/perf/Documentation/perf-record.txt

@@ -88,6 +88,20 @@ OPTIONS
           If you want to profile write accesses in [0x1000~1008), just set
           'mem:0x1000/8:w'.
 
+        - a BPF source file (ending in .c) or a precompiled object file (ending
+          in .o) selects one or more BPF events.
+          The BPF program can attach to various perf events based on the ELF section
+          names.
+
+          When processing a '.c' file, perf searches an installed LLVM to compile it
+          into an object file first. Optional clang options can be passed via the
+          '--clang-opt' command line option, e.g.:
+
+            perf record --clang-opt "-DLINUX_VERSION_CODE=0x50000" \
+                        -e tests/bpf-script-example.c
+
+          Note: '--clang-opt' must be placed before '--event/-e'.
+
 	- a group of events surrounded by a pair of brace ("{event1,event2,...}").
 	  Each event is separated by commas and the group should be quoted to
 	  prevent the shell interpretation.  You also need to use --group on