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

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Message-ID: <87poijkg25.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Wed, 15 Feb 2017 10:34:58 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        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

Arnaldo Carvalho de Melo <acme@...nel.org> writes:

> Em Tue, Feb 14, 2017 at 03:24:16PM +0200, Alexander Shishkin escreveu:
>> 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.
>> 
>> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
>> Reported-by: Andi Kleen <ak@...ux.intel.com>
>
> Yeah, I saw that as well, and Andi told me about this limitation, so,
> for quite a while now, everytime I need to test PT on the only machine I
> have with it, I have to stop my kvm sessions :-\
>
> Thanks for working on this!

Well, we can't make the limitation go away. You'll still have to stop
kvms to get any 'meaningful' PT data.

Regards,
--
Alex