From: Alexei Starovoitov <ast@plumgrid.com>
To: xiakaixu <xiakaixu@huawei.com>
Cc: davem@davemloft.net, acme@kernel.org, mingo@redhat.com,
a.p.zijlstra@chello.nl, masami.hiramatsu.pt@hitachi.com,
jolsa@kernel.org, wangnan0@huawei.com,
linux-kernel@vger.kernel.org, pi3orama@163.com,
hekuang@huawei.com
Subject: Re: [PATCH v3 3/3] samples/bpf: example of get selected PMU counter value
Date: Thu, 23 Jul 2015 19:23:28 -0700 [thread overview]
Message-ID: <55B1A1A0.7000808@plumgrid.com> (raw)
In-Reply-To: <55B19AB9.3090100@huawei.com>
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@huawei.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.
prev parent reply other threads:[~2015-07-24 2:23 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-23 9:42 [PATCH v3 0/3] bpf: Introduce the new ability of eBPF programs to access hardware PMU counter Kaixu Xia
2015-07-23 9:42 ` [PATCH v3 1/3] bpf: Add new bpf map type to store the pointer to struct perf_event Kaixu Xia
2015-07-23 22:54 ` Alexei Starovoitov
2015-07-24 2:22 ` xiakaixu
2015-07-24 2:26 ` Alexei Starovoitov
2015-08-03 9:38 ` Peter Zijlstra
2015-07-23 9:42 ` [PATCH v3 2/3] bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter Kaixu Xia
2015-07-23 22:56 ` Alexei Starovoitov
2015-07-24 1:57 ` xiakaixu
2015-08-03 9:34 ` Peter Zijlstra
2015-08-03 10:32 ` xiakaixu
2015-07-23 9:42 ` [PATCH v3 3/3] samples/bpf: example of get selected PMU counter value Kaixu Xia
2015-07-23 22:59 ` Alexei Starovoitov
2015-07-24 1:54 ` xiakaixu
2015-07-24 2:23 ` Alexei Starovoitov [this message]
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