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

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Message-ID: <20170215133721.GX6515@twins.programming.kicks-ass.net>
Date:   Wed, 15 Feb 2017 14:37:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
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

On Wed, Feb 15, 2017 at 03:05:10PM +0200, Alexander Shishkin wrote:
> 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.

kvm_intel has a vmm_exclusive parameter, if you set that to 0 is
_should_ behave much better.