linux-kernel - Re: [PATCH v3 3/3] samples/bpf: example of get selected PMU counter value

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Date:	Thu, 23 Jul 2015 19:23:28 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	xiakaixu <xiakaixu@...wei.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 v3 3/3] samples/bpf: example of get selected PMU counter
 value

On 7/23/15 6:54 PM, xiakaixu wrote:
> 于 2015/7/24 6:59, Alexei Starovoitov 写道:
>> On 7/23/15 2:42 AM, Kaixu Xia wrote:
>>> This is a simple example and shows how to use the new ability
>>> to get the selected Hardware PMU counter value.
>>>
>>> Signed-off-by: Kaixu Xia <xiakaixu@...wei.com>
>> ...
>>> +struct bpf_map_def SEC("maps") my_map = {
>>> +    .type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
>>> +    .key_size = sizeof(int),
>>> +    .value_size = sizeof(unsigned long),
>>> +    .max_entries = 32,
>>> +};
>>
>> wait. how did it work here? value_size should be u32.
>
> I tested the whole thing on ARM board. You are ringt, it
> should be u32.
> When create the array map, we choose the array->elem_size as
> round_up(attr->value_size, 8), why 8?

because from user space point of view we're storing FDs
which are u32, but kernel stores pointers.
but round_up(attr->value_size, 8) is done because there
can be 8 byte fields in there and we have 8-byte load/store insns.
So whether pointer is 32 or 64-bit they still fit.
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