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Date:   Thu, 22 Aug 2019 11:05:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Yonghong Song <yhs@fb.com>, Daniel Xu <dxu@dxuuu.xyz>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        Andrii Nakryiko <andriin@fb.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "acme@kernel.org" <acme@kernel.org>,
        Alexei Starovoitov <ast@fb.com>,
        "alexander.shishkin@linux.intel.com" 
        <alexander.shishkin@linux.intel.com>,
        "jolsa@redhat.com" <jolsa@redhat.com>,
        "namhyung@kernel.org" <namhyung@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl
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On Thu, Aug 22, 2019 at 07:54:16AM +0000, Song Liu wrote:
> Hi Peter, 
> 
> > On Aug 22, 2019, at 12:47 AM, Peter Zijlstra <peterz@infradead.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()?

It really shouldn't be hard; and I'm failing to see why kprobes are
special.