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From: Stephane Eranian <eranian@google.com>
Date: Thu, 15 Jun 2017 23:52:07 -0700
Message-ID: <CABPqkBQCbiW=b0kXXCUwh_e1FFTNLgHrA8en2Q1_Ov-W9-NBcQ@mail.gmail.com>
Subject: Re: [PATCH 0/5] perf: add support for capturing skid IP
To: Andi Kleen <ak@linux.intel.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, "mingo@elte.hu" <mingo@elte.hu>,
        "Liang, Kan" <kan.liang@intel.com>, Jiri Olsa <jolsa@redhat.com>
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Andi,

On Thu, Jun 15, 2017 at 4:18 PM, Andi Kleen <ak@linux.intel.com> wrote:
>> Looking at this approach, the user interface is straightforward,
>> implementation in the x86 code is a bit more hairy because of the way
>> the branch_stack is captured, via the cpuc->lbr_entries. If you assume
>> that SKID_IP cannot be used with any other branch stack mode, then it
>> is easy. It becomes messy if you don't.
>
> That should be fine. After all if you have real LBRs you don't need
> the skid IP.
>
Yes, you still do. This is not the same thing. LBR captures only taken branches.
I care about taken AND non-taken branches and I don't want to sample on a
non-taken event, assuming it is available.

You need to inject the skid ip into the LBR stack somehow. Either
directly in the
registers or in the cpuc->lbr_entries. The injection can only happen
on interrupt
whereas the LBR captures 32 branches. So you'd have the skid ip info
only for the
most recent branch in the stack.