linux-kernel - Re: [PATCH v1 1/5] perf/core: Define the common branch type classification

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Message-ID: <20170406065850.kziehynhayizgtk2@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 08:58:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jin Yao <yao.jin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
        linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v1 1/5] perf/core: Define the common branch type
 classification

On Tue, Apr 04, 2017 at 11:18:05AM -0300, Arnaldo Carvalho de Melo wrote:
> Adding the perf kernel maintainers to the CC list.

Thanks.

> Em Fri, Mar 31, 2017 at 11:18:38PM +0800, Jin Yao escreveu:
> > It is often useful to know the branch types while analyzing branch
> > data. For example, a call is very different from a conditional branch.
> > 
> > Currently we have to look it up in binary while the binary may later
> > not be available and even the binary is available but user has to take
> > some time. It is very useful for user to check it directly in perf
> > report.
> > 
> > Perf already has support for disassembling the branch instruction
> > to get the branch type. The branch type is defined in lbr.c.
> > 
> > To keep consistent on kernel and userspace and make the classification
> > more common, the patch adds the common branch type classification
> > in perf_event.h.
> > 
> > Since the disassembling of branch instruction needs some overhead,
> > a new PERF_SAMPLE_BRANCH_TYPE_SAVE is introduced to indicate if it
> > needs to disassemble the branch instruction and record the branch
> > type.

I don't get it. Why is the kernel interface mucked with for a user-space
feature?

That's wrong.