netdev - Re: [PATCH bpf v3 0/5] libbpf: remove two dependencies on Linux

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Message-ID: <CAPhsuW4C58jOwVN+pdOZa7O0m+RM8NDQfkaotkwxwL375Of4ug@mail.gmail.com>
Date:   Tue, 16 Apr 2019 14:25:27 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...el.com>
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        bruce.richardson@...el.com, ciara.loftus@...el.com,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        xiaolong.ye@...el.com, ferruh.yigit@...el.com,
        qi.z.zhang@...el.com, georgmueller@....net
Subject: Re: [PATCH bpf v3 0/5] libbpf: remove two dependencies on Linux

On Tue, Apr 16, 2019 at 5:58 AM Magnus Karlsson
<magnus.karlsson@...el.com> wrote:
>
> This patch set fixes one bug and removes two dependencies on Linux
> kernel headers from the XDP socket code in libbpf. A number of people
> have pointed out that these two dependencies make it hard to build the
> XDP socket part of libbpf without any kernel header dependencies. The
> two removed dependecies are:
>
> * Remove the usage of likely and unlikely (compiler.h) in xsk.h. It
>   has been reported that the use of these actually decreases the
>   performance of the ring access code due to an increase in
>   instruction cache misses, so let us just remove these.
>
> * Remove the dependency on barrier.h as it brings in a lot of kernel
>   headers. As the XDP socket code only uses two simple functions from
>   it, we can reimplement these. As a bonus, the new implementation is
>   faster as it uses the same barrier primitives as the kernel does
>   when the same code is compiled there. Without this patch, the user
>   land code uses lfence and sfence on x86, which are unnecessarily
>   harsh/thorough.
>
> In the process of removing these dependencies a missing barrier
> function for at least PPC64 was discovered. For a full explanation on
> the missing barrier, please refer to patch 1. So the patch set now
> starts with two patches fixing this. I have also added a patch at the
> end removing this full memory barrier for x86 only, as it is not
> needed there.
>
> Structure of the patch set:
> Patch 1-2: Adds the missing barrier function in kernel and user space.
> Patch 3-4: Removes the dependencies
> Patch 5: Optimizes the added barrier from patch 2 so that it does not
>          do unnecessary work on x86.
>
> v2 -> v3:
> * Added missing memory barrier in ring code
> * Added an explanation on the three barriers we use in the code
> * Moved barrier functions from xsk.h to libbpf_util.h
> * Added comment on why we have these functions in libbpf_util.h
> * Added a new barrier function in user space that makes it possible to
>   remove the full memory barrier on x86.
>
> v1 -> v2:
> * Added comment about validity of ARM 32-bit barriers.
>   Only armv7 and above.
>
> /Magnus
>
> Magnus Karlsson (5):
>   xsk: fix XDP socket ring buffer memory ordering
>   libbpf: fix XDP socket ring buffer memory ordering
>   libbpf: remove likely/unlikely in xsk.h
>   libbpf: remove dependency on barrier.h in xsk.h
>   libbpf: optimize barrier for XDP socket rings

For the set:

Acked-by: Song Liu <songliubraving@...com>

>
>  net/xdp/xsk_queue.h         | 56 +++++++++++++++++++++++++++++++++++++++++----
>  tools/lib/bpf/libbpf_util.h | 30 ++++++++++++++++++++++++
>  tools/lib/bpf/xsk.h         | 22 +++++++++++++-----
>  3 files changed, 98 insertions(+), 10 deletions(-)
>
> --
> 2.7.4