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 10:39:58 +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 10:22, Alexei Starovoitov 写道:
> On 7/22/15 7:12 PM, xiakaixu wrote:
>> 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];
>>    ...
>> }
> 
> yes. the only thing needed is to add:
> if (index >= array->map.max_entries)
>     return -E2BIG;
> before accessing 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
> 
> correct.
> 
>>   really want is map key. So I'm not sure ARG_ANYTHING is suitable.
>>   You know ARG_ANYTHING is not checked enough in verifier.
> 
> why? during perf_event_array creation time we check that key_size == u32
> so we can accept any integer.
> ARG_PTR_TO_MAP_KEY forces program author to use stack instead of
> passing index directly. Direct index is obviously faster.

Copy that. We will follow them in V3.
> 
> 
> .
> 


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