Re: [PATCH v2] bpf: Remove config check to enable bpf support for branch records

[kajoljain <kjain@linux.ibm.com>]

On 11/20/21 4:15 AM, Andrii Nakryiko wrote:
> On Fri, Nov 19, 2021 at 8:08 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>
>> On 11/19/21 10:35 AM, kajoljain wrote:
>>> On 11/19/21 4:18 AM, Andrii Nakryiko wrote:
>>>> On Thu, Nov 18, 2021 at 5:10 AM Kajol Jain <kjain@linux.ibm.com> wrote:
>>>>>
>>>>> Branch data available to bpf programs can be very useful to get
>>>>> stack traces out of userspace application.
>>>>>
>>>>> Commit fff7b64355ea ("bpf: Add bpf_read_branch_records() helper")
>>>>> added bpf support to capture branch records in x86. Enable this feature
>>>>> for other architectures as well by removing check specific to x86.
>>>>> Incase any platform didn't support branch stack, it will return with
>>>>> -EINVAL.
>>>>>
>>>>> Selftest 'perf_branches' result on power9 machine with branch stacks
>>>>> support.
>>>>>
>>>>> Before this patch changes:
>>>>> [command]# ./test_progs -t perf_branches
>>>>>   #88/1 perf_branches/perf_branches_hw:FAIL
>>>>>   #88/2 perf_branches/perf_branches_no_hw:OK
>>>>>   #88 perf_branches:FAIL
>>>>> Summary: 0/1 PASSED, 0 SKIPPED, 1 FAILED
>>>>>
>>>>> After this patch changes:
>>>>> [command]# ./test_progs -t perf_branches
>>>>>   #88/1 perf_branches/perf_branches_hw:OK
>>>>>   #88/2 perf_branches/perf_branches_no_hw:OK
>>>>>   #88 perf_branches:OK
>>>>> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>>>>>
>>>>> Selftest 'perf_branches' result on power9 machine which doesn't
>>>>> support branch stack
>>>>>
>>>>> After this patch changes:
>>>>> [command]# ./test_progs -t perf_branches
>>>>>   #88/1 perf_branches/perf_branches_hw:SKIP
>>>>>   #88/2 perf_branches/perf_branches_no_hw:OK
>>>>>   #88 perf_branches:OK
>>>>> Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED
>>>>>
>>>>> Fixes: fff7b64355eac ("bpf: Add bpf_read_branch_records() helper")
>>>>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>>>>> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
>>>>> ---
>>>>>
>>>>> Tested this patch changes on power9 machine using selftest
>>>>> 'perf branches' which is added in commit 67306f84ca78 ("selftests/bpf:
>>>>> Add bpf_read_branch_records()")
>>>>>
>>>>> Changelog:
>>>>> v1 -> v2
>>>>> - Inorder to add bpf support to capture branch record in
>>>>>    powerpc, rather then adding config for powerpc, entirely
>>>>>    remove config check from bpf_read_branch_records function
>>>>>    as suggested by Peter Zijlstra
>>>>
>>>> what will be returned for architectures that don't support branch
>>>> records? Will it be zero instead of -ENOENT?
>>>
>>> Hi Andrii,
>>>       Incase any architecture doesn't support branch records and if it
>>> tries to do branch sampling with sample type as
>>> PERF_SAMPLE_BRANCH_STACK, perf_event_open itself will fail.
>>>
>>> And even if, perf_event_open succeeds  we have appropriate checks in
>>> bpf_read_branch_records function, which will return -EINVAL for those
>>> architectures.
>>>
>>> Reference from linux/kernel/trace/bpf_trace.c
>>>
>>> Here, br_stack will be empty, for unsupported architectures.
>>>
>>> BPF_CALL_4(bpf_read_branch_records, struct bpf_perf_event_data_kern *, ctx,
>>>          void *, buf, u32, size, u64, flags)
>>> {
>>> .....
>>>       if (unlikely(flags & ~BPF_F_GET_BRANCH_RECORDS_SIZE))
>>>               return -EINVAL;
>>>
>>>       if (unlikely(!br_stack))
>>>               return -EINVAL;
>>
>> In that case for unsupported archs we should probably bail out with -ENOENT here
>> as helper doc says '**-ENOENT** if architecture does not support branch records'
>> (see bpf_read_branch_records() doc in include/uapi/linux/bpf.h).
> 
> Yep, I think so too.
> 

Hi Andrii/Daniel,
     I agree, changing return type to -ENOENT make sense, I will update
in next version of this patch.

Thanks,
Kajol Jain

>>
>>> ....
>>> }
>>>
>>> Thanks,
>>> Kajol Jain