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From: Stephane Eranian <eranian@google.com>
Date: Wed, 8 Jun 2016 00:27:33 -0700
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Subject: Re: [PATCH 1/3] perf/x86/intel: output LBR support statement after validation
To: Andi Kleen <ak@linux.intel.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>,
        LKML <linux-kernel@vger.kernel.org>, "x86@kernel.org" <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, "Yan, Zheng" <zheng.z.yan@intel.com>,
        Kan Liang <kan.liang@intel.com>, Peter Zijlstra <peterz@infradead.org>
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Andi,

On Sun, Jun 5, 2016 at 6:59 PM, Andi Kleen <ak@linux.intel.com> wrote:
>
> > It is not because you force LBR to ring3 only that you do not capture
> > kernel addresses in the FROM field.
> > Keep in mind that LBR priv level filtering applies to the target of
> > the branch and not the source. You might
> > still get a kernel address if returning from kernel. Now, in callstack
> > mode, I think the return branch is never
> > actually recorded in the LBR, it just causes a pop, so theoretically
> > this should not happen. I'd like to be
> > 100% sure of that, though.
>
> Far branches shouldn't be included in call stack LBR. Don't think
> there is any other situation where the ring 0 address could leak either.
>
Ok, so you're saying that syscall and int are not causing LBR callstack to
record an entry. If that is the case, then a rfi should not cause a pop of the
last LBR entry. Is that right?

>
> -Andi
> --
> ak@linux.intel.com -- Speaking for myself only