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Date:   Thu, 20 Sep 2018 18:24:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Wei Wang <wei.w.wang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, pbonzini@redhat.com, kan.liang@intel.com,
        mingo@redhat.com, rkrcmar@redhat.com, like.xu@intel.com,
        jannh@google.com, arei.gonglei@huawei.com
Subject: Re: [PATCH v3 4/5] KVM/x86/vPMU: Add APIs to support host
 save/restore the guest lbr stack
Message-ID: <20180920162407.GA24124@hirez.programming.kicks-ass.net>
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 <20180920153035.GB10360@tassilo.jf.intel.com>
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On Thu, Sep 20, 2018 at 08:30:35AM -0700, Andi Kleen wrote:
> > +int intel_pmu_enable_save_guest_lbr(struct kvm_vcpu *vcpu)
> > +{
> > +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> > +	struct perf_event *event;
> > +	struct perf_event_attr attr = {
> > +		.type = PERF_TYPE_RAW,
> > +		.size = sizeof(attr),
> > +		.pinned = true,
> > +		.exclude_host = true,
> > +		.sample_type = PERF_SAMPLE_BRANCH_STACK,
> > +		.branch_sample_type = PERF_SAMPLE_BRANCH_CALL_STACK |
> > +				      PERF_SAMPLE_BRANCH_USER |
> > +				      PERF_SAMPLE_BRANCH_KERNEL,
> 
> I think that will allocate an extra perfmon counter, right? 

I throught the same too, but I think the exclude_host/guest, whichever
is the right one makes that work for regular counters too.

That code is a wee bit magical and I didn't take the time to reverse
engineer that. It most certainly needs a comment.