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    SubjectRe: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot
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    > On Sep 2, 2021, at 3:53 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
    >
    > 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?

    Hmm.. This is still relevant, but not very accurate. I guess we should
    provide more information, like "For more information about branches before
    the trigger point, this should be called early in the BPF program".

    Song


    >
    >> + * 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.
    >
    > [...]

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