linux-kernel - Re: [PATCH v2 4/5] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter

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Date:	Thu, 23 Jul 2015 10:12:53 +0800
From:	xiakaixu <xiakaixu@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	<davem@...emloft.net>, <acme@...nel.org>, <mingo@...hat.com>,
	<a.p.zijlstra@...llo.nl>, <masami.hiramatsu.pt@...achi.com>,
	<jolsa@...nel.org>, <wangnan0@...wei.com>,
	<linux-kernel@...r.kernel.org>, <pi3orama@....com>,
	<hekuang@...wei.com>
Subject: Re: [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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