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Date: Thu, 29 Jun 2017 10:12:33 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Kyle Huey <me@kylehuey.com>, Vince Weaver <vincent.weaver@maine.edu>,
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Subject: Re: [PATCH] perf/core: generate overflow signal when samples are
 dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we
 entered the kernel in the "skid" region)
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* Mark Rutland <mark.rutland@arm.com> wrote:

> It still seems wrong to make up data, though.

So what we have here is a hardware quirk: we asked for user-space samples, but 
didn't get them and we cannot expose the kernel-internal address.

The question is, how do we handle the hardware quirk. Since we cannot fix the 
hardware on existing systems there's really just two choices:

 - Lose the sample (and signal it as a lost sample)

 - Keep the sample but change the sensitive kernel-internal address to something 
   that is not sensitive: 0 or -1 works, but we could perhaps also return a 
   well-known user-space address such as the vDSO syscall trampoline or such?

there's no other option really.

I'd lean towards Vince's take: losing samples is more surprising than getting the 
occasional sample with some sanitized data in it.

If we make the artificial data still a meaningful user-space address, related to 
kernel entries, then it might even be a bonus, as users would learn to recognize 
it as: 'oh, skid artifact, I know about that'.

Thanks,

	Ingo