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

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Message-ID: <20190822090555.GJ2349@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Aug 2019 11:05:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
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

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@...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()?

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