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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 2 Sep 2021 15:53:20 -0700
Message-ID: <CAEf4BzZLSs3ejyVLPMORd_GPCYNE8Jz4M6=4wxzR576Vag-+-A@mail.gmail.com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot
To:     Song Liu <songliubraving@fb.com>
Cc:     bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Ziljstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Kajol Jain <kjain@linux.ibm.com>,
        Kernel Team <kernel-team@fb.com>
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On Thu, Sep 2, 2021 at 9:58 AM Song Liu <songliubraving@fb.com> wrote:
>
> Introduce bpf_get_branch_snapshot(), which allows tracing pogram to get
> branch trace from hardware (e.g. Intel LBR). To use the feature, the
> user need to create perf_event with proper branch_record filtering
> on each cpu, and then calls bpf_get_branch_snapshot in the bpf function.
> On Intel CPUs, VLBR event (raw event 0x1b00) can be use for this.
>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>  include/uapi/linux/bpf.h       | 22 ++++++++++++++++++++++
>  kernel/bpf/trampoline.c        |  3 ++-
>  kernel/trace/bpf_trace.c       | 33 +++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h | 22 ++++++++++++++++++++++
>  4 files changed, 79 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 791f31dd0abee..c986e6fad5bc0 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4877,6 +4877,27 @@ union bpf_attr {
>   *             Get the struct pt_regs associated with **task**.
>   *     Return
>   *             A pointer to struct pt_regs.
> + *
> + * long bpf_get_branch_snapshot(void *entries, u32 size, u64 flags)
> + *     Description
> + *             Get branch trace from hardware engines like Intel LBR. The
> + *             branch trace is taken soon after the trigger point of the
> + *             BPF program, so it may contain some entries after the

This part is a leftover from previous design, so not relevant anymore?

> + *             trigger point. The user need to filter these entries
> + *             accordingly.
> + *
> + *             The data is stored as struct perf_branch_entry into output
> + *             buffer *entries*. *size* is the size of *entries* in bytes.
> + *             *flags* is reserved for now and must be zero.
> + *
> + *     Return
> + *             On success, number of bytes written to *buf*. On error, a
> + *             negative value.
> + *
> + *             **-EINVAL** if arguments invalid or **size** not a multiple
> + *             of **sizeof**\ (**struct perf_branch_entry**\ ).
> + *
> + *             **-ENOENT** if architecture does not support branch records.

[...]