linux-kernel - Re: [PATCH 1/3] perf/x86/intel: output LBR support statement after validation

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

Andi,

On Sun, Jun 5, 2016 at 6:59 PM, Andi Kleen <ak@...ux.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@...ux.intel.com -- Speaking for myself only