linux-kernel - Re: [PATCH v3 4/5] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack

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Date:   Thu, 20 Sep 2018 18:24:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Wei Wang <wei.w.wang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, pbonzini@...hat.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com
Subject: Re: [PATCH v3 4/5] KVM/x86/vPMU: Add APIs to support host
 save/restore the guest lbr stack

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.