linux-kernel - Re: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX operation is on

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Date:   Tue, 14 Feb 2017 15:02:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX
 operation is on

On Tue, Feb 14, 2017 at 03:24:16PM +0200, Alexander Shishkin wrote:
> On systems where PT does not coexist with VMX, users get confused when
> PT turns up with no data because they forgot they're running a kvm
> session at the same time.
> 
> This patch adds a preemptive check for any active VMX operations that
> will fail event creation. This does not provide any guarantees or
> protection against racing with a kvm starting in parallel, but is
> intended to serve as a hint for the user. If VMXON happens after an
> event had been created, the event will still produce an empty trace.
> 

Would it not be more sensible to write fake PT packets in
intel_pt_handle_vmx() to demarcate VMX regions in the trace?