netdev - Re: [net-next PATCH V3 3/5] samples/bpf: add a README file to get users started

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Date:	Wed, 27 Apr 2016 11:16:34 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:	netdev@...r.kernel.org, linux-kbuild@...r.kernel.org,
	bblanco@...mgrid.com, borkmann@...earbox.net,
	alexei.starovoitov@...il.com, brouer@...hat.com
Subject: Re: [net-next PATCH V3 3/5] samples/bpf: add a README file to get
 users started

On Wed, 27 Apr 2016 14:05:22 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> On 2016/04/27 09:30AM, Jesper Dangaard Brouer wrote:
> > Getting started with using examples in samples/bpf/ is not
> > straightforward.  There are several dependencies, and specific
> > versions of these dependencies.
> > 
> > Just compiling the example tool is also slightly obscure, e.g. one
> > need to call make like:
> > 
> >  make samples/bpf/
> > 
> > Do notice the "/" slash after the directory name.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> > ---
> >  samples/bpf/README.rst |   75 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 75 insertions(+)
> >  create mode 100644 samples/bpf/README.rst  
> 
> Thanks for adding this! A few nits...

I would prefer if we could apply this patchset and you could followup
with a patch with your nits...

> > 
> > diff --git a/samples/bpf/README.rst b/samples/bpf/README.rst
> > new file mode 100644
> > index 000000000000..1fa157db905b
> > --- /dev/null
> > +++ b/samples/bpf/README.rst
> > @@ -0,0 +1,75 @@
> > +eBPF sample programs
> > +====================
> > +
> > +This kernel samples/bpf directory contains a mini eBPF library, test  
> 	^^^^^^^^^^^^^^^^^^
> 'This directory contains' should suffice.

The reason I formulated it like this, was that people will often hit
this kind of documentation when searching google.


> > +stubs, verifier test-suite and examples for using eBPF.
> > +
> > +Build dependencies
> > +==================
> > +
> > +Compiling requires having installed:
> > + * clang >= version 3.4.0
> > + * llvm >= version 3.7.1
> > +
> > +Note that LLVM's tool 'llc' must support target 'bpf', list with command::
> > +
> > + $ llc --version  
> 
> 'llc --version | grep bpf' is probably simpler?

I wanted to give people the impression of how the output looks like.
 
> > + LLVM (http://llvm.org/):
> > +  LLVM version 3.x.y
> > +  [...]
> > +  Host CPU: xxx
> > +
> > +  Registered Targets:
> > +    [...]
> > +    bpf        - BPF (host endian)
> > +    bpfeb      - BPF (big endian)
> > +    bpfel      - BPF (little endian)
> > +    [...]
> > +
> > +Kernel headers
> > +--------------
> > +
> > +There are usually dependencies to header files of the current kernel.
> > +To avoid installing devel kernel headers system wide, as a normal
> > +user, simply call::
> > +
> > + make headers_install
> > +
> > +This will creates a local "usr/include" directory in the git/build top
> > +level directory, that the make system automatically pickup first.
> > +
> > +Compiling
> > +=========
> > +
> > +For compiling goto kernel top level build directory and run make like::  
> 
> For building the BPF samples, issue the below command from the kernel 
> root directory:

I like your formulation better, but it it worth a respin of the entire
patchset? 

Notice you need the extra "::" ending of the paragraph, to make this
document format nicely with RST (ReStructuredText).

The a README with a .rst suffix will be picked up by github and
displayed as the doc for the directory. Thus I also made sure it
"compiles" with the rst tools. E.g see how samples/pktgen gets auto
documented and nicely formatted via github (scroll down):
 https://github.com/torvalds/linux/tree/master/samples/pktgen

> > +
> > + make samples/bpf/
> > +
> > +Do notice the "/" slash after the directory name.
> > +
> > +Manually compiling LLVM with 'bpf' support
> > +------------------------------------------
> > +
> > +Since version 3.7.0, LLVM adds a proper LLVM backend target for the
> > +BPF bytecode architecture.
> > +
> > +By default llvm will build all non-experimental backends including bpf.
> > +To generate a smaller llc binary one can use::
> > +
> > + -DLLVM_TARGETS_TO_BUILD="BPF;X86"  
> 
> Is the X86 target really needed?

I'm not sure, but if you want to use clang/llc for something else it is
useful, and the example usage of the ";" separator syntax makes it
worth including as an example.

> > +
> > +Quick sniplet for manually compiling LLVM and clang
> > +(build dependencies are cmake and gcc-c++)::
> > +
> > + $ git clone http://llvm.org/git/llvm.git
> > + $ cd llvm/tools
> > + $ git clone --depth 1 http://llvm.org/git/clang.git
> > + $ cd ..; mkdir build; cd build
> > + $ cmake .. -DLLVM_TARGETS_TO_BUILD="BPF;X86"  
> 					    ^^^
> Here too.
> 
> - Naveen
> 
> > + $ make -j $(getconf _NPROCESSORS_ONLN)
> > +
> > +It is also possible to point make to the newly compiled 'llc' command
> > +via redefining LLC on the make command line::
> > +
> > + make samples/bpf/ LLC=~/git/llvm/build/bin/llc
> > +
> >   
> 



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer