linux-kernel - Re: [PATCH 0/5] bpf: Add user-space-publisher ringbuffer map type

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Date:   Mon, 8 Aug 2022 11:57:53 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     David Vernet <void@...ifault.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, john.fastabend@...il.com, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, kpsingh@...nel.org, sdf@...gle.com,
        jolsa@...nel.org, tj@...nel.org, joannelkoong@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] bpf: Add user-space-publisher ringbuffer map type

Hi David,

On Mon, Aug 8, 2022 at 8:52 AM David Vernet <void@...ifault.com> wrote:
>
> This patch set defines a new map type, BPF_MAP_TYPE_USER_RINGBUF, which
> provides single-user-space-producer / single-kernel-consumer semantics over
> a ringbuffer.  Along with the new map type, a helper function called
> bpf_user_ringbuf_drain() is added which allows a BPF program to specify a
> callback with the following signature, to which samples are posted by the
> helper:
>
> void (struct bpf_dynptr *dynptr, void *context);
>
> The program can then use the bpf_dynptr_read() or bpf_dynptr_data() helper
> functions to safely read the sample from the dynptr. There are currently no
> helpers available to determine the size of the sample, but one could easily
> be added if required.
>
> On the user-space side, libbpf has been updated to export a new
> 'struct ring_buffer_user' type, along with the following symbols:
>
> struct ring_buffer_user *
> ring_buffer_user__new(int map_fd,
>                       const struct ring_buffer_user_opts *opts);
> void ring_buffer_user__free(struct ring_buffer_user *rb);
> void *ring_buffer_user__reserve(struct ring_buffer_user *rb, uint32_t size);
> void *ring_buffer_user__poll(struct ring_buffer_user *rb, uint32_t size,
>                              int timeout_ms);
> void ring_buffer_user__discard(struct ring_buffer_user *rb, void *sample);
> void ring_buffer_user__submit(struct ring_buffer_user *rb, void *sample);
>
> These symbols are exported for inclusion in libbpf version 1.0.0.
>
> Note that one thing that is not included in this patch-set is the ability
> to kick the kernel from user-space to have it drain messages. The selftests
> included in this patch-set currently just use progs with syscall hooks to
> "kick" the kernel and have it drain samples from a user-producer
> ringbuffer, but being able to kick the kernel using some other mechanism
> that doesn't rely on such hooks would be very useful as well. I'm planning
> on adding this in a future patch-set.
>

This could be done using iters. Basically, you can perform draining in
bpf_iter programs and export iter links as bpffs files. Then to kick
the kernel, you simply just read() the file.

> Signed-off-by: David Vernet <void@...ifault.com>
> --
>
> David Vernet (5):
>   bpf: Clear callee saved regs after updating REG0
>   bpf: Define new BPF_MAP_TYPE_USER_RINGBUF map type
>   bpf: Add bpf_user_ringbuf_drain() helper
>   bpf: Add libbpf logic for user-space ring buffer
>   selftests/bpf: Add selftests validating the user ringbuf
>
>  include/linux/bpf.h                           |   6 +-
>  include/linux/bpf_types.h                     |   1 +
>  include/uapi/linux/bpf.h                      |   9 +
>  kernel/bpf/helpers.c                          |   2 +
>  kernel/bpf/ringbuf.c                          | 232 ++++++-
>  kernel/bpf/verifier.c                         |  73 ++-
>  tools/include/uapi/linux/bpf.h                |   9 +
>  tools/lib/bpf/libbpf.c                        |  11 +-
>  tools/lib/bpf/libbpf.h                        |  19 +
>  tools/lib/bpf/libbpf.map                      |   6 +
>  tools/lib/bpf/libbpf_probes.c                 |   1 +
>  tools/lib/bpf/ringbuf.c                       | 214 +++++++
>  .../selftests/bpf/prog_tests/user_ringbuf.c   | 592 ++++++++++++++++++
>  .../selftests/bpf/progs/user_ringbuf_fail.c   | 174 +++++
>  .../bpf/progs/user_ringbuf_success.c          | 227 +++++++
>  .../testing/selftests/bpf/test_user_ringbuf.h |  28 +
>  16 files changed, 1579 insertions(+), 25 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/user_ringbuf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/user_ringbuf_fail.c
>  create mode 100644 tools/testing/selftests/bpf/progs/user_ringbuf_success.c
>  create mode 100644 tools/testing/selftests/bpf/test_user_ringbuf.h
>
> --
> 2.30.2
>