linux-kernel - Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Date:	Wed, 12 Aug 2015 06:15:05 -0700
From:	Brenden Blanco <bblanco@...il.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	llvm-dev@...ts.llvm.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	pi3orama <pi3orama@....com>,
	Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [llvm-dev] llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf:
 Introduce function for outputing data to perf event

Hi Wangnan, I've been authoring the BCC development, so I'll answer
those specific questions.
>
>
> Could you please give us further information about your clang rewriter?
> I guess you need a new .so when injecting those code into kernel?

The rewriter runs all of its passes in a single process, creating no
files on disk and having no external dependencies in terms of
toolchain.
1. Entry point: bpf_module_create() - C API call to create module, can
take filename or directly a c string with the full contents of the
program
2. Convert contents into a clang memory buffer
3. Set up a clang driver::CompilerInvocation in the style of the clang
interpreter example
4. Run a rewriter pass over the memory buffer file, annotating and/or
doing BPF specific magic on the input source
 a. Open BPF maps with a call to bpf_create_map directly
 b. Convert references to map operations with the specific FD of the new map
 c. Convert arguments to bpf_probe_read calls as needed
 d. Collect the externed function names to avoid section() hack in the language
5. Re-run the CompilerInvocation on the modified sources
6. JIT the llvm::Module to bpf arch
7. Load the resulting in-memory ".o" to bpf_prog_load, keeping the FD
alive in the compiler process
8. Attach the FD as necessary to perf events, socket, tc, etc.
9. goto 1

The above steps are captured in the BCC github repo in src/cc, with
the clang specific bits inside of the frontends/clang subdirectory.

> I'm not sure. Our target platform should be embedded devices like
> smartphone.
> Bringing full clang/llvm environment there is not acceptable.

The artifact from the build process of BCC is a shared library, which
has the clang/llvm .a embedded within them. It is not yet a single
binary, but not unfeasible to make it so. The clang toolchain itself
does not need to exist on the target. I have not attempted to
cross-compile BCC to any architecture, currently x86_64 only.

If you have more BCC specific questions not involving clang/llvm,
perhaps you can ping Alexei/myself off of the llvm-dev list, in case
this discussion is not relevant to them.
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