linux-kernel - Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add PERF_EVENT_IOC_QUERY

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Date:   Thu, 22 Aug 2019 07:54:16 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Yonghong Song <yhs@...com>, Daniel Xu <dxu@...uu.xyz>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Andrii Nakryiko <andriin@...com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        Alexei Starovoitov <ast@...com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "Arnaldo Carvalho de Melo" <acme@...hat.com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl

Hi Peter, 

> On Aug 22, 2019, at 12:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Aug 21, 2019 at 06:43:49PM +0000, Yonghong Song wrote:
>> On 8/21/19 11:31 AM, Peter Zijlstra wrote:
> 
>>> So extending PERF_RECORD_LOST doesn't work. But PERF_FORMAT_LOST might
>>> still work fine; but you get to implement it for all software events.
>> 
>> Could you give more specifics about PERF_FORMAT_LOST? Googling 
>> "PERF_FORMAT_LOST" only yields two emails which we are discussing here :-(
> 
> Look at what the other PERF_FORMAT_ flags do? Basically it is adding a
> field to the read(2) output.

Do we need to implement PERF_FORMAT_LOST for all software events? If user
space asks for PERF_FORMAT_LOST for events that do not support it, can we
just fail sys_perf_event_open()?

Thanks,
Song