linux-kernel - Re: [RFC PATCH tip 5/5] tracing filter examples in BPF

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Date:	Tue, 3 Dec 2013 17:21:37 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Jonathan Corbet <corbet@....net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Jovi Zhangwei <jovi.zhangwei@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH tip 5/5] tracing filter examples in BPF

On Tue, Dec 3, 2013 at 4:35 PM, Jonathan Corbet <corbet@....net> wrote:
> On Mon,  2 Dec 2013 20:28:50 -0800
> Alexei Starovoitov <ast@...mgrid.com> wrote:
>
>>  GCC-BPF backend is available on github
>>  (since gcc plugin infrastructure doesn't allow for out-of-tree backends)
>
> Do you have a pointer to where this backend can be found?  I've
> done a bit of digging around but seem to be unable to find it…

here it is:
https://github.com/iovisor/bpf_gcc/commit/9e7223f8f09c822ecc6e18309e89a574a23dbf63

It's quite small comparing to normal backend:
 config.me                          |    3 +
 config.sub                         |   12 +-
 gcc/common/config/bpf/bpf-common.c |   39 ++
 gcc/config.gcc                     |   18 +
 gcc/config/bpf/bpf-modes.def       |   22 +
 gcc/config/bpf/bpf-protos.h        |   60 ++
 gcc/config/bpf/bpf.c               | 1063 ++++++++++++++++++++++++++++++++++++
 gcc/config/bpf/bpf.h               |  746 +++++++++++++++++++++++++
 gcc/config/bpf/bpf.md              |  895 ++++++++++++++++++++++++++++++
 gcc/config/bpf/linux.h             |   67 +++
 10 files changed, 2922 insertions(+), 3 deletions(-)

Instruction emission is in bpf.md in functions like:
(define_insn "adddi3"
  [(set (match_operand:DI 0 "int_reg_operand" "=r, r")
  (plus:DI (match_operand:DI 1 "int_reg_operand" "0, 0")
    (match_operand:DI 2 "int_reg_or_const_operand" "r, K")))]
  ""
  "@
   BPF_INSN_ALU(BPF_ADD, %0, %2), // %0 += %2
   BPF_INSN_ALU_IMM(BPF_ADD, %0, %2), // %0 += %2"
[(set_attr "type" "arith,arith") ])

So it takes C and emits BPF_* macros from include/linux/bpf.h
tools/bpf/trace/ examples were generated by it.
Eventually it will be changed to emit binary hex directly, but macros
are much easier to understand for now.

Thanks
Alexei
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