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

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Message-ID: <87d1ejk3jt.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Wed, 15 Feb 2017 15:05:10 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        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

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Feb 15, 2017 at 10:34:58AM +0200, Alexander Shishkin wrote:
>
>> Well, we can't make the limitation go away. You'll still have to stop
>> kvms to get any 'meaningful' PT data.
>
> Why would you need to stop all VMs in order to get your !VM data? Sure,
> you get black holes where the VM runs, but we should be able to see
> everything else.

No, what I mean is that if you run kvm prior to starting perf record,
which I assume is the case for acme, your entire session is a black
hole. The VMXON happens pretty early on, you can open /dev/kvm,
ioctl(KVM_CREATE_VM) on it and that will do a VMXON already.

The problem is that PT (on BDW) doesn't trace inside VM root mode, not
just between VM entry/VM exit.

Regards,
--
Alex