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    SubjectRe: [PATCH v2 4/5] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter
    于 2015/7/23 9:14, Alexei Starovoitov 写道:
    > On 7/22/15 1:09 AM, Kaixu Xia wrote:
    >> According to the perf_event_map_fd and key, the function
    >> bpf_perf_event_read() can convert the corresponding map
    >> value to the pointer to struct perf_event and return the
    >> Hardware PMU counter value.
    >>
    >> The key can't be passed to bpf_perf_event_read() directly
    >> because the function argument constraint is lacked.
    >
    > I don't understand above sentence.
    >
    >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
    >> index 69a1f6b..e3bb181 100644
    >> --- a/include/uapi/linux/bpf.h
    >> +++ b/include/uapi/linux/bpf.h
    >> @@ -250,6 +250,8 @@ enum bpf_func_id {
    >> * Return: 0 on success
    >> */
    >> BPF_FUNC_get_current_comm,
    >> +
    >> + BPF_FUNC_perf_event_read, /* u64 bpf_perf_event_read(&map, &key) */
    >
    > no need for extra empty line.
    >
    >> +
    >> +static u64 bpf_perf_event_read(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
    >> +{
    >> + struct bpf_map *map = (struct bpf_map *) (unsigned long) r1;
    >> + void *key = (void *) (unsigned long) r2;
    >> + struct perf_event *event;
    >> + void *ptr;
    >> +
    >> + if (map->map_type != BPF_MAP_TYPE_PERF_EVENT_ARRAY)
    >> + return -EINVAL;
    >
    > please check this statically in verifier instead of in run-time.
    >
    >> +
    >> + rcu_read_lock();
    >
    > unnecessary.
    >
    >> + ptr = map->ops->map_lookup_elem(map, key);
    >> + rcu_read_unlock();
    >> + if (!ptr || !(*(unsigned long *)ptr))
    >> + return -EBADF;
    >
    > all these casts can be removed. First cast of 'r1' into
    > perf_event_array will be enough.

    So you mean like this?

    u64 bpf_perf_event_read(u64 r1, u64 index,...)
    {
    struct bpf_perf_event_array *array = (void *) (long) r1;
    struct perf_event *event;
    ...
    event = array->events[index];
    ...
    }
    >
    >> +const struct bpf_func_proto bpf_perf_event_read_proto = {
    >> + .func = bpf_perf_event_read,
    >> + .gpl_only = false,
    >> + .ret_type = RET_INTEGER,
    >> + .arg1_type = ARG_CONST_MAP_PTR,
    >> + .arg2_type = ARG_PTR_TO_MAP_KEY,
    >
    > make it arg2_type = ARG_ANYTHING then you'll just index
    > into array the way prog_array does and similar to bpf_tail_call.

    ARG_ANYTHING means any (initialized) argument is ok, but we here
    really want is map key. So I'm not sure ARG_ANYTHING is suitable.
    You know ARG_ANYTHING is not checked enough in verifier.

    Thanks.
    >
    >
    > .
    >




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