[dpdk-dev] [PATCH v1] net/tap: explain how to compile eBPF C file

[Wiles, Keith]

> On Jun 11, 2018, at 11:06 AM, Ophir Munk <ophirmu at mellanox.com> wrote:
> 
> This commit explains how to manually compile the C source file
> tap_bpf_program.c into an ELF file using the clang compiler.
> The code in tap_bpf_program.c requires definitions found in iproute2
> source code. This commit suggests cloning the iproute2 git tree and
> include its path in the clang command. It also adds inclusion of file
> bpf_api.h (required for eBPF definitions) which is located in iproute2
> source tree. For more details refer to TAP documentation.
> This commit is related to commits [1] and [2].

Normally I would have suggested that eBPF be disable in the TAP driver as it requires external code and programs, but that ship has sailed.

I would like to see building the tap_bpf_program.o as a target in the Makefile, this way the developer can just run the ‘make bpf_program’ target and it would be simpler and less error prone.
> 
> [1] commit cdc07e83bb24 ("net/tap: add eBPF program file")
> [2] commit aabe70df73a3 ("net/tap: add eBPF bytes code")
> 
> Signed-off-by: Ophir Munk <ophirmu at mellanox.com>
> ---
> doc/guides/nics/tap.rst           | 21 +++++++++++++++++----
> drivers/net/tap/tap_bpf_program.c |  5 +++++
> 2 files changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/doc/guides/nics/tap.rst b/doc/guides/nics/tap.rst
> index 2714868..af6c534 100644
> --- a/doc/guides/nics/tap.rst
> +++ b/doc/guides/nics/tap.rst
> @@ -234,13 +234,26 @@ C functions under different ELF sections.
> 
> 2. Install ``LLVM`` library and ``clang`` compiler versions 3.7 and above
> 
> -3. Compile ``tap_bpf_program.c`` via ``LLVM`` into an object file::
> +3. The code in ``tap_bpf_program.c`` requires definitions found in iproute2
> +source code.
> 
> -    clang -O2 -emit-llvm -c tap_bpf_program.c -o - | llc -march=bpf \
> -    -filetype=obj -o <tap_bpf_program.o>
> +Clone the iproute2 git tree and make it accessible to the build environment, say
> +under directory ``<iproute2_root_tree>`` ::
> +
> +    git clone https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/ \
> +    <iproute2_root_tree>
> +
> +The code in ``tap_bpf_program.c`` must include file ``bpf_api.h`` which is
> +located under ``<iproute2_root_tree>`` directory. This file contains eBPF
> +related definitions.
> 
> +4. Compile ``tap_bpf_program.c`` via ``LLVM`` into an object file::
> +
> +    clang -I <iproute2_root_tree>/iproute2/include \
> +    -O2 -emit-llvm -c tap_bpf_program.c -o - | llc -march=bpf \
> +    -filetype=obj -o <tap_bpf_program.o>
> 
> -4. Use a tool that receives two parameters: an eBPF object file and a section
> +5. Use a tool that receives two parameters: an eBPF object file and a section

What ‘tool’ should be used here? objdump?

> name, and prints out the section as a C array of eBPF instructions.
> Embed the C array in your TAP PMD tree.
> 
> diff --git a/drivers/net/tap/tap_bpf_program.c b/drivers/net/tap/tap_bpf_program.c
> index 1cb7382..60b069b 100644
> --- a/drivers/net/tap/tap_bpf_program.c
> +++ b/drivers/net/tap/tap_bpf_program.c
> @@ -17,6 +17,11 @@
> #include <linux/bpf.h>
> 
> #include "tap_rss.h"
> +/*
> + * bpf_api.h file is located under iproute2
> + * tree, see TAP documentation.
> + */
> +#include "bpf_api.h"
> 
> /** Create IPv4 address */
> #define IPv4(a, b, c, d) ((__u32)(((a) & 0xff) << 24) | \
> -- 
> 1.8.3.1
> 

Regards,
Keith