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Date:   Thu, 20 Sep 2018 08:30:35 -0700
From:   Andi Kleen <ak@linux.intel.com>
To:     Wei Wang <wei.w.wang@intel.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        pbonzini@redhat.com, kan.liang@intel.com, peterz@infradead.org,
        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: <20180920153035.GB10360@tassilo.jf.intel.com>
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> +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? 

So if the guest wants to use all perfmon counters we would start to 
multiplex, which would be bad

Would need to fix perf core to not allocate a perfmon counter in
this case, if no period and no event count is requested.

-Andi